DE10025562C1 - Production of a nitride layer on a substrate used in the production of laser diodes comprises vaporizing gallium atoms and aluminum and/or indium atoms and irradiating substrate during vaporization with nitrogen atoms - Google Patents

Abstract

Production of a nitride layer on a substrate comprises vaporizing gallium atoms and aluminum and/or indium atoms on the substrate; and irradiating the surface of the substrate during vaporization with nitrogen atoms. At the start of the growing process, the flow of the gallium atoms is continuously increased from a first value to a larger second value. An Independent claim is also included for a quasi substrate (6) comprising a base substrate (2) with a gallium nitride layer (7). Preferred Features: The nitride layer is a gallium nitride layer and the substrate is made of sapphire. The gallium layer is grown along the c-axis of the substrate. The substrate temperature is between room temperature and 900 deg C. The ratio of gallium atoms to nitrogen ions is 0.1-30.

Description

The invention relates to a method for producing a Nitride layer on a substrate.

The invention further relates to a quasi-substrate according to claim 12 or 14.

In connection with the production of laser diodes with the Layer sequence GaN, InGaN and GaN is for example off No. 5,742,628 discloses GaN layers on a substrate to grow. The GaN layer grown on the substrate bil detects the basis for the subsequent shift sequence through which the laser diode structure is formed.

In C.-W. Jeon et al., Thin solid films, 270 (1995) pp. 16-21 describes the production of a GaN layer on a Si (111) substrate. The GaN layer is grown epitaxially by simultaneously applying Ga vapor and N ₂ ⁺ ions to the substrate, an increase in the layer quality being achieved by reducing the ion flow.

The deposition of GaN is due to the large differences in the lattice constants of GaN and possible substrates sword. The lattice mismatch between GaN and sapphire be carries about 16%, for example, that between GaN and SiC 3.5%.

Due to the different lattice constants of GaN and ei The underlying substrate is created in the GaN layer Tensions that are reduced by dislocations. These Dislocations start growing from the substrate direction and impair the function below Layers. In particular, the dislocations act as non-  radiating recombination centers as well as scattering centers for Charge carrier. As a result, the dislocations reduce the pho clay yield. The transfers also promote training local increases in indium concentration in one the InGaN layer adjacent to the GaN layer. This will make the Band gap reduced, causing the blur of the emission waves length of the laser light generated is increased.

The invention is based on this prior art the task is based on a method of manufacture as possible dislocation-free nitride layer on a substrate ben.

This object is achieved in that atoms, selected from the group of elements Al, Ga and In on which Substrate are evaporated and that the surface of the sub strats during the vapor deposition of the atoms with nitrogen ions is irradiated.

These measures allow the tension in the up growing nitride layer predominantly through zero-dimensional Dismantle point defects without getting stuck in the growth direction Effective transfers arise. This gives you nitride Layers with higher kri compared to the prior art stallin quality, which can be used as quasi substrates for the manufacturer position of components. Under a quasi-substrate is different from a conventional homogeneous and single crystal substrate a substrate in the form of a basic sub strates with a on a base substrate surface brought another layer to understand.

In one embodiment of the method, the kinetic Energy of the nitrogen ions chosen so that this between the displacement energy of the nitrogen ions in the solid material and the displacement energy on the surface of the nitride Layer.  

So the energy of the nitrogen ions is so high that it diffuse enough on the surface of the nitride layers dieren and thereby the designated places in Kri can find stall bars. At the same time is the energy the nitrogen ions so low that the radiation damage on the top atomic layer of the nitride layer remains limited.

By using the method according to the invention ent are in particular quasi substrates with a GaN layer a low surface density of dislocations in the GaN Layer. The low areal density of dislocations makes itself felt  especially with a characteristic Rutherford backscatter Spectrum noticeable. In particular, it assigns gallium part of the he by Rutherford backscatter spectrometry averaged spectrum between those by the substra surface and the maxima caused by the layer surface a minimum that is less than 10% of the unguided spec at this energy. Such a spectrum is characteristic of a low areal density at offset conditions and defects. Further advantageous embodiments of the Invention are the subject of the dependent claims.

The invention is explained in more detail below with reference to the beige added drawing explained. Show it:

Fig. 1, a conventional quasi-substrate with a device;

Fig. 2 is a cross section through a quasi-substrate accelerator as the invention;

FIG. 3 shows the device used to grow the GaN layer; FIG.

Fig. 4 is a perspective view of a sample in a X-ray diffractometer;

Fig. 5 is a θ-2θ diffraction spectrum of a GaN layer of a quasi substrate according to the invention;

Fig. 6 shows a rocking curve of a GaN layer on egg nem quasi substrate according to the invention;

Fig. 7a and b a {113} pole figure of -Röntgen-c-Sapphire as the related {112} pole figure of the GaN grown thereon thin layer;

Fig. 8 is a guided and unguided Rutherford backscattering spectrum of a GaN layer of a quasi substrate;

9 is a diagram that provides a ramp growth rates based on the temperature of the effusion cell.

FIG. 10a and b Rocking curves of produced without growth rates ramp and with growth rates ramp quasi substrates;

Fig. 11a and b Rutherford backscatter spectra of growth rate ramp without growth and with quasi substrates produced with growth rate ramp; and

Fig. 12a and b atomic force micrographs of GaN layers of produced without growth rates ramp and with growth rates ramp quasi substrates.

Fig. 1 shows a conventional quasi-substrate 1 having on egg nem base substrate 2, a GaN layer 3, the actual device 4 connects with its layer sequence on. Materials such as Al ₂ O ₃ , SiC, Si, Mg ₂ O ₄ , ZnO and GaAs are usually used for the base substrate 2 . The deposition of the GaN layer is associated with difficulties due to the large differences in the lattice constants of GaN and the materials that are possible for the base substrate 2 . The lattice mismatch between GaN and Al ₂ O ₃ is, for example, 16%, that between GaN and SiC is about 3.5%. Due to the different lattice constants of GaN and the materials that are possible for the base substrate 2, stresses generally form in the GaN layer 3 , which are reduced by thread-like dislocations 5 running in the growth direction. The filamentous conditions 5 can continue in the area of the component 4 . There, they promote the excitation of phonons when the charge carriers are recombined in component 4 and reduce the photon yield. If the component 4 is a laser diode with the layer sequence GaN-InGaN-GaN, locally increased concentrations of indium can form at the thread-like dislocations 5 . As a result, the band gap is reduced at this point, which leads to a spreading of the blur of the emission wavelength of the laser diode.

In the quasi-substrate 6 shown in FIG. 2, a GaN layer 7 is formed between the base substrate 2 and the component 4 . The GaN layer 7 is produced by evaporating gallium onto the base substrate 2 with simultaneous irradiation with nitrogen ions. The GaN layer 7 produced in this way has no thread-like dislocations 5 . Accordingly, the component 4 has a significantly greater efficiency compared to the prior art.

In Fig. 3 is illustrated an apparatus for practicing the method. For evaporation of the gallium atoms on the base substrate 2 , an effusion cell 8 is provided, which is operated in the temperature range between 970 and 1050 ° C. This results in a maximum gallium flow of 2 × 10 ¹⁵ atoms / cm ² / s. The temperature of the base substrate 2 is chosen between room temperature and 900 ° C, preferably between 600 and 800 ° C.

To generate the nitrogen ions, an ion source 9 is seen before. The ion source 9 generates a nitrogen ion flow in which the nitrogen ions have kinetic energies between 10 and 1500 eV. In order to ensure that fabric-ions on the surface of the GaN layer 7, the stick can diffuse sufficiently long, the energy of the nitrogen ions should preferably be above the displacement energy of the embroidery cloth ions on the surface of the GaN layer 7 are to be grown. On the other hand, in order to keep the radiation damage as low as possible, the kinetic energy of the nitrogen ions should be as low as possible or only slightly above the displacement energy of the gallium atoms in the GaN layer 7 . The kinetic energy of the nitrogen ions should therefore be in the range from 15 to 40 eV.

The ion current density of the currents generated by the ion source 9 should be below 750 µA / cm ² . This results in a ratio of the number of nitrogen ions to gallium atoms between 0.1 and 30.

The device shown in FIG. 3 was used to grow on a base substrate 2 made of Al ₂ O ₃ GaN layers 7 with a thickness of 300 nm.

For structural analysis of the GaN layers 7 grown on the base substrate 2 , X-ray diffraction measurements were carried out, the results of which are presented using the following figures. The angle information used here will first be explained with reference to FIG. 4.

In FIG. 4, a sample 10 is illustrated, che their Oberflä 11, an incident X-ray 12 is incident. The incident X-ray beam 12 is reflected on the grating planes of the sample 10 and forms a diffracted X-ray beam 13 . The incident x-ray beam 12 and the diffracted x-ray beam 13 span an incidence plane in which a diffractometer circuit 14 of a detector is located. During the measurement, the sample 10 can now be rotated about different spatial axes. A spatial direction running in the plane of incidence and along the upper surface 11 is referred to as the χ axis. The direction of space parallel to the surface normal of the surface 11 is called the ϕ-axis 16 . The direction oriented perpendicular to the plane of incidence, which runs parallel to the surface 11 , finally bears the designation 2 θ / θ axis or ω axis 17 .

It is clear from FIG. 5 that the thin GaN layers 7 deposited on c-sapphire under 25 eV ion irradiation have a pronounced crystalline structure with a clear preferred direction. The diffraction reflections contained in FIG. 5 can be clearly assigned to the wurtzitic or also hexagonal GaN phase.

The small half width of the ω spectrum in FIG. 6 shows that the GaN layer 7 has only a very low mosaicity. The high crystalline quality of the GaN layer 7 follows from this. That the GaN layer has indeed grown epitaxially on the base substrate 2 can be seen from the X-ray pole figures shown in FIGS. 7a and b. For Recordin in Fig. 7a and b shown pole figures me the detector is moved in a diffraction peak and the sample 10 is tilted gradually to the χ-axis 15. With a fixed χ angle, the sample 10 is then rotated through 360 ° about the ϕ axis 16 . In the pole figures shown in FIGS . 7a and b, the length of the arrows 18 is a measure of the χ angle.

The six pole density maxima 19 of the {112} pole figure of the GaN layer 7 , which reflects the hexagonal symmetry of the GaN layer 7 , are of extremely narrow half-width, comparable to the pole density maxima 20 of the {113} pole figure of the base substrate 2 , which is shown in Fig. 7a. The following epitaxial relationships can be derived from the corresponding pole figures of the base substrate 2 and the GaN layer 7 , which correspond to those from the literature: GaN (0001) ∥ Al ₂ O ₃ (0001) and GaN [2110] ∥ Al ₂ O ₃ [1100]. These relationships apply to the growth of the GaN layer 7 on c-sapphire. The GaN layer 7 therefore grows c-axis oriented on c-sapphire.

Rutherford backscattering spectrometry with ion lattice guidance is particularly suitable for the detection of defects in crystals that are not too small, and was additionally used for structural analysis by X-ray diffraction. The investigations carried out here were carried out with the help of He ²⁺ ions with an energy of 2.5 MeV. In FIG. 8, the number of backscattering events Ge Gen the energy of the back-scattered helium ions applied. In FIG. 8 is both an unguided spectrum 21 as well as a run spectrum 22 is illustrated. In the case of a guided spectrum, the substrate is oriented towards the ion beam in such a way that the ions are guided (channeled) through the crystal lattice of the substrate. In the case of an unguided spectrum, substrate and ion beam are arranged with respect to one another such that this guidance does not occur.

The unguided spectrum 21 essentially reflects the stoichiometry of the layers. The guided spectrum in lattice channels of the quasi 6 may penetrate far into the quasi-substrate 6 because the helium ions in the acquisition on the substrate without being scattered back, has the spectrum led 22 generally to a lower count rate. This applies in particular when the lattice channels of the quasi substrate 6 are not disturbed by dislocations or defects. The distance between the unguided spectrum 21 and the guided spectrum 22 is therefore a measure of the number of defects.

Because of the high mass of the gallium atoms, the helium ions scattered back on the gallium atoms in the GaN layer 7 have a particularly high kinetic energy. The unguided spectrum 21 and the guided spectrum 22 therefore have particularly high count rates in the range of 2 MeV. In particular, the guided spectrum 22 has a first maximum 23 , which mainly results from the scattering of the helium ions on the gallium atoms on the surface of the GaN layer 7 . However, the first maximum 23 does not only originate from the backscattering of the helium ions on the gallium atoms on the surface of the GaN layer 7 , but also under certain circumstances from the scattering of gallium droplets on the surface of the GaN layer 7 . Furthermore, the guided spectrum 22 has a maximum 24 , which is based on the backscattering of helium ions in the region of the interface between GaN layer 7 and the base substrate 2 . Because of the large lattice mismatch between the GaN layer 7 and the quasi-substrate 6 made of sapphire, a large number of dislocations is present in the area of the interface between the GaN layer 7 and the base substrate 2 , which only occurs with increasing layer thickness of the GaN layer 7 is reduced by mutually canceling the dislocations.

Between the first maximum 23 and the second maximum 24 of the guided spectrum 22 there is a minimum 25 in which the number of backscatter events is only 2.4% of the number of backscatter events of the unguided spectrum 21 at this energy. The low value of this minimum 25 demonstrates the good crystallinity of the GaN layer 7 and the small number of defects and dislocations in the GaN layer 7 . From Be meaning is also that the critical angle for the ion grid guide at the selected energy of the helium ions of 2.5 MeV is about 0.7 °. This is also an indication of good crystallinity of the GaN layer 7 .

The crystalline quality of the GaN layer 7 can be sustainably improved by continuously increasing the gallium flow from an initially low value to an end value at the beginning of the growth process at a constant substrate temperature and with a constant nitrogen ion flow. Since at the beginning of the growth process the gallium flow is the variable that determines the growth rate, it is justified to speak of a growth rate ramp.

The measurements presented below were carried out on two different sample types. In both samples, a 300 nm thick GaN layer 7 was deposited on c-sapphire. In one sample type, the effusion cell temperature was increased from 970 to 1020 ° C over a period of 20 minutes at a constant substrate temperature of 700 ° C and then continued to grow at this temperature. The ramp executed via the temperature of the effu sion cell 8 is shown in FIG. 9. The gallium flows associated with the respective temperature (TEZ) of the effusion cell 8 are listed in Table 1.

Table 1

This process is briefly referred to below as production Ramp designated.

In the second type of sample, on the other hand, the growth process was started immediately with a temperature of the effusion cell 8 of 1020 ° C. The temperature of the base substrate 2 was 700 ° C.

. In the Fig 10a and Fig. 10b rocking curves shown it can be seen that the half-value width can be reduced by almost a factor of 2 by the application of the ramp. This means that the tilting of the GaN crystallites around the growth direction can be significantly reduced by manufacturing with a ramp.

Furthermore, a phase selection effect occurs when using the ramp. By recording ϕ-scans of the cubic (111) reflections and the hexagonal (101) reflections, it was found that the GaN layers 7 deposited without a ramp accounted for a significant portion of the cubic phase due to the occasional or frequent occurrence of stacking errors as well as cubic own twins.

In addition, the use of the ramp also has a partial impact on the depth distribution of the defects. This becomes clear from the Rutherford backscatter spectra shown in FIGS. 11a and 11b. From Fig. 11a shows that the second peak 24 is the guided spectrum 22 almost 50% of the unguided spectrum 21st There is no minimum. Rather, the number of backscatter results slowly decreases to 13.9% towards the first maximum 23 . The cubic phase identified in the X-ray measurements leads in many cases to large-angle scattering and thus to an increase in the backscattering yield. The guided spectrum 22 shown in FIG. 11b of a GaN layer 7 produced with a ramp is characterized overall by a substantially lower backscattering yield. In addition, the guided spectrum 22 in FIG. 11b has a pronounced first maximum 23 and a pronounced second maximum 24 with an intermediate minimum 25 .

Finally, the use of the ramp has effects not only with regard to the structure, but also with regard to the topography of the GaN layer 7 , as the scanning force microscopic recordings in FIGS . 12a and 12b prove. The surface of the GaN layer 7 produced without a ramp is significantly rougher than the surface of the GaN layer 7 produced with a ramp. This can also be seen in the mean square roughness determined for these surfaces (RMS roughness, root mean square roughness). The mean square roughness is 6.4 nm for the area shown in FIG. 12, but only 1.2 nm for the surface shown in FIG. 13.

Furthermore, it can be seen from FIGS. 12a and 12b that the use of the ramp causes a transition from island growth to two-dimensional layer growth.

The method described here can also be used if the materials used for the base substrate 2, such as Al ₂ O ₃ , SiC, Si, Mg ₂ O ₄ , ZnO and GaAs. The layers applied to SiC have even better qualities due to the lower lattice mismatch compared to sapphire. In particular, the method is also suitable for growing a GaN layer on r-sapphire. The GaN layer grows a-axis oriented.

By using the described method, GaN layers 7 can be produced which, despite a small layer thickness of less than 300 nm, have a surface density of the thread-like dislocations 5 of less than 10 ⁹ cm ^-2 .

In principle, the process presented here can also be used to produce layers of GaN-based compound semiconductors. Such a layer generally has the composition Al _x Ga _y In _z N with x + y + z = 1 and 0 ≦ x, y, z ≦ 1. Thus, the method can also be used for AlGaN-, InGaN- and Al GaInN- Create layers.

Reference symbol list

1

conventional quasi-substrate

2nd

Base substrate

3rd

GaN layer

4th

Component

5

threadlike dislocations

6

Quasi substrate

7

GaN layer

8th

Effusion cell

9

Hollow anode source

10th

sample

11

surface

12th

incident x-ray

13

diffracted x-ray

14

Diffractometer circuit

15

χ axis

16

ϕ axis

17th

ω axis

18th

arrow

19th

Pole density maxima

20th

Pole density maxima

21st

unguided spectrum

22

guided spectrum

23

first maximum

24th

second maximum

25th

minimum

Claims (14)

1. A method for producing a nitride layer ( 7 ) on a substrate ( 2 ), in which gallium atoms and atoms, selected from the group of the elements Al and In, are evaporated onto the substrate ( 2 ) and the surface of the Substrate ( 2 ) is irradiated with nitrogen ions during the vapor deposition of the atoms, characterized in that at the beginning of the growth process the flow of gallium atoms is continuously increased from a first value to a second, larger value. 2. The method according to claim 1, characterized in that the nitride layer is a GaN layer ( 7 ) and the substrate ( 2 ) made of sapphire. 3. The method according to claim 2, characterized in that the GaN layer ( 7 ) is grown along the c-axis of the substrate ( 2 ). 4. The method according to any one of claims 1 to 3, characterized in that the substrate temperature between room temperature and 900 ° C lies. 5. The method according to any one of claims 1 to 4, characterized in that the gallium flow is below 2 × 10 ¹⁵ atoms / cm ² / s ¹ . 6. The method according to any one of claims 1 to 5, characterized in that the ratio of gallium atoms to nitrogen ions between between 0.1 and 30. 7. The method according to any one of claims 1 to 6, characterized in that the kinetic energy of the nitrogen ions between the Ver storage energy in the GaN layer ( 7 ) and the displacement energy on the surface of the GaN layer ( 7 ). 8. The method according to claim 7, characterized in that the kinetic energy of the nitrogen ions between 10 and 1500 eV lies. 9. The method according to claim 8, characterized in that the kinetic energy of nitrogen ions between 15 eV and 40 eV is. 10. The method according to any one of claims 1 to 9, characterized in that the flow of gallium atoms continuously in the range from 0.6 × 10 ¹⁴ atoms / cm ² / s to 3.3 × 10 ¹⁴ atoms / cm ² / s is increased. 11. The method according to claim 10, characterized in that the flow of gallium atoms between 15 and Is increased 45 minutes. 12. Quasi-substrate with a base substrate ( 2 ) which has a substrate surface on which a gallium-containing layer ( 7 ) is applied with a layer surface, characterized in that the gallium assignable part of the spectrum determined by Rutherford backscattering spectrometry ( 22 ) between the maxima ( 23 , 24 ) generated by the substrate surface and the layer surface has a minimum ( 25 ) which is less than 10% of the unguided spectrum ( 21 ) at this energy. 13. quasi-substrate according to claim 12, characterized in that the minimum is less than 5% of the unguided spectrum ( 21 ). 14. Quasi-substrate with a base substrate ( 2 ) having a substrate surface on which a GaN layer ( 7 ) is brought up, characterized in that the areal density of the threadlike dislocations ( 5 ) in the GaN layer ( 7 ) below 10 ⁹ cm ^-2 lies.