Method of growing a semiconductor layer

Details Method of growing a semiconductor layer Method of growing a semiconductor layer

2001-06-18

2002-12-31

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Forming from vapor or gaseous state

Including change in a growth-influencing parameter

C117S089000, C117S094000, C117S101000, C117S106000, C117S952000

Reexamination Certificate

active

06500258

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a molecular beam epitaxy (MBE) method for the epitaxial growth of Group III nitride semiconductor materials such as, for example, GaN or other members of the (Ga, Al, In)N material family.
2. Description of the Related Art
The epitaxial growth of Group III nitride semiconductor materials on a substrate can be effected by molecular beam epitaxy (MBE) or by chemical vapour deposition (CVD) which is sometimes known as Vapour Phase Epitaxy (VPE).
CVD (or VPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the other elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
In contrast to CVD, MBE in carried out in a high vacuum environment. In the case of MBE as applied to the GaN system, an ultra-high vacuum (UHV) environment, typically around 1×10
−3
Pa, is used. Ammonia or another nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and a species providing gallium and, possibly, indium and/or aluminium are supplied from appropriate sources within heated effusion cells fitted with controllable shutters to control the amounts of the species supplied into the MBE chamber during the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place. The ammonia and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
At present, the majority of growth of high quality GaN layers is carried out using the metal-organic chemical vapour deposition (MOCVD) process. The MOCVD process allows good control of the growth of the nucleation layer and of the annealing of the nucleation layer. Furthermore, the MOCVD process allows growth to occur at a V/III ratio well in excess of 1000:1. The V/III ratio is the molar ratio of the group V element to the Group III element during the growth process. A high V/III ratio is preferable, since this allows a higher substrate temperature to be used which in turn leads to a higher quality GaN layer.
At present, growing high quality GaN layers by MBE is more difficult than growing such layers by MOCVD. The principal difficulty is in supplying sufficient nitrogen during the growth process; it is difficult to obtain a V/III ratio of 10:1 or greater. The two commonly used sources of nitrogen In the MBE growth of nitride layers are plasma excited molecular nitrogen or ammonia.
GaN has a lattice constant of around 0.45 nm. There is a lack of suitable substrates that, are lattice-matched to GaN, so GaN is generally grown onto either a sapphire substrate or a silicon carbide substrate. There is a large mis-match between the lattice constant of GaN and the lattice constant of sapphire or silicon carbide, and there is also a considerable difference in thermal properties, such as the thermal expansion coefficient, between the GaN layer and the substrate. It is therefore necessary to provide a thin initial nucleation layer on the substrate in order to grow a high quality GaN layer on sapphire or silicon carbide.
I. Akasaki and I. Amano report, in “Japanese Journal of Applied Physics” Vol. 36, pp5393-5408 (1997), that a thin AlN layer, deposited at a low growth temperature, can be used as a nucleation layer to promote the growth of a GaN layer by metal organic chemical vapour deposition (MOCVD) process on a sapphire or silicon carbide substrate. U.S. Pat. No. 5,290,393 discloses the use of a GaN nucleation layer, again deposited at a low growth temperature, for promoting the growth of a GaN layer using MOCVD.
U.S. Pat. No. 5,385,862 discloses a further method of growing a single crystal GaN film on a sapphire substrate using MBE. In this method, a nucleation layer is grown on the substrate at a growth temperature of 400° C. or lower. Furthermore, the V/III ratio of this method is very small, being less than 5:1, so that the subsequent GaN layer is restricted to a growth temperature of lower than 900° C. GaN layers grown by this method have electron mobilities at room temperature of less than 100 cm
2
V
−1
s
−1
.
Further prior art methods of growing GaN on a sapphire or silicon carbide substrate are reported by Z. Yang et al in “Applied Physics Letters” Vol. 67, pp1686-1688 (1995), and by N. Grandjean et al in “Applied Physics Letters” Vol. 71, p240-242 (1997). In both of these methods a GaN nucleation layer is initially grown on the substrate, after which the GaN layer is grown.
Although the provision of a nucleation layer does reduce the effect of the lattice and thermal mis-match between a GaN layer and a sapphire or silicon carbide substrate, the effects of the lattice and thermal mis-match are not eliminated completely. Moreover it is difficult and time-consuming to optimise the nucleation layer so as to obtain the highest possible quality GaN, and the step of growing the nucleation layer adds to the complexity of the growth process. It is accordingly desirable to use a GaN substrate for the growth of an epitaxial GaN layer.
A GaN substrate for use in the epitaxial growth of GaN can have two possible forms—a GaN substrate can be a “free-standing” substrate or a “template” substrate. A free-standing GaN substrate consists solely of GaN, and is formed by, for example, a GaN crystal. A template GaN substrate consists of a thick epitaxial layer of GaN grown on a base substrate of, for example, sapphire or silicon carbide. The thick epitaxial layer is grown on the base substrate by any suitable technique, such as metal-organic vapour phase epitaxy (MOVPE) or hydride vapour phase epitaxy (HVPE). Compared with the nucleation layers mentioned above, the epitaxial layer of a GaN template substrate is much thicker than a nucleation layer, for example having a thickness in the range 5 &mgr;m-100 &mgr;m.
M. Kamp et al report, in “Mat Res Soc Proc”, Vol. 449, p161 (1997), the growth of a GaN layer by MBE on a free-standing GaN substrate. They obtain good quality GaN, having a photoluminescence (PL) linewidth with a FWHM of 0.5 meV and a dislocation density in the range 10
2
to 10
3
cm
−2
. However, Kamp et al achieve a growth temperature of only 750° C.
WO 97/13891 discloses a method of epitaxial growth of a nitride semiconductor layer (GaN or Ga(Al,In)N) on a single crystal GaN or Ga(Al,In)
N
. This document is primarily directed to the way in which the substrate is produced, and teaches disposing a solution of Ga or Ga,Al,In in a heated nitrogen atmosphere so as to grow a bulk crystal of GaN or Ga(Al,In)N.
Once the bulk crystal has been grown, it is used as the substrate in an epitaxial growth process. The document speculates that it would be possible to grow an epitaxial layer on the substrate by MBE in the temperature range 500-900° C. however, the document contains no teaching as to how an MBE growth temperature for GaN of 900° C. could be achieved.
The growth of GaN on a GaN template substrate has been reported by, for example, W. C. Hughes et al in “J. Vac. Sci. Technol B” Vol. 13, p1571 (1995). In this report, GaN is grown by MBE with plasma excited molecular nitrogen used as the source of nitrogen for the MBE growth process. Other reports of the MBE growth of GaN on a GaN template substrate have been made by E. J., Tarsa et al in “Journal of Applied-Physics”, Vol. 82, p5472 (1997); by H. Sakai et al in “Japanese Journal of Applied Physics”, Vol. 34, L1429 (1995); by M. A.

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