Growth of GaN on sapphire with MSE grown buffer layer

A method of fabricating a gallium nitride or like epilayer on sapphire is disclosed wherein a buffer layer is grown on the sapphire substrate by magnetron sputter epitaxy (MSE); and then the gallium nitride epilayer is formed on the buffer layer, preferably by molecular beam epitaxy.

FIELD OF THE INVENTION
 This invention relates to the growth of gallium nitride and like materials
 on a sapphire substrate.
 BACKGROUND OF THE INVENTION
 In recent years there has been tremendous interest in GaN based III-N
 materials. A dramatic improvement in the material quality has led to the
 development of high brightness light emitting diodes, and more recently
 "blue" laser diodes. There has also been a dramatic improvement in the
 performance of high power microwave metal-semiconductor field-effect
 transistors and modulation doped field-effect transistors based on these
 materials.
 Crucial to all these applications is the growth of material with high
 crystalline quality and of high purity. Various techniques have been used
 to grow GaN including metalorganic vapor phase epitaxy (MOVPE), plasma
 molecular beam epitaxy (plasma MBE), ammonia molecular beam epitaxy
 (ammonia-MBE), also referred to as reactive molecular beam epitaxy, and
 magnetron sputter epitaxy (MSE). Typically GaN epilayers are grown on
 sapphire substrates, which are highly lattice mismatched, necessitating
 the predeposition of a thin (.about.500 .ANG.) buffer/nucleation layer of
 either GaN or AlN. The observed electrical and optical properties of the
 resulting GaN layers is strongly dependent on the dislocation density and
 of the overall impurity content.
 Using these growth techniques, room temperature electron mobilities for
 MOVPE-grown silicon doped GaN layers are typically reported in the range
 of 350-600 cm.sup.2 /V s. The highest room temperature mobility ever
 reported for GaN was 900 cm.sup.2 /V s deposited by MOVPE for a 4 .mu.m
 thick layer. In contrast, the highest room temperature mobility for
 plasma-MBE grown GaN is around 300 cm.sup.2 /V s and for ammonia-MBE is
 350 cm.sup.2 /V s.
 SUMMARY OF THE INVENTION
 According to the present invention there is provided a method of
 fabricating a gallium nitride or like epilayer on sapphire, comprising the
 steps of providing a sapphire substrate, growing a nucleation buffer layer
 on said sapphire substrate by magnetron sputter epitaxy (MSE) and
 subsequently forming said gallium nitride epilayer on said buffer layer.
 Using this method, silicon doped GaN epilayers having room temperature
 electron mobilities&gt;550 cm.sup.2 /V s can be grown on grown on (0001
 )sapphire. Unlike other growth techniques, the initial buffer/nucleation
 layer, preferably of AlN (aluminum nitride), is grown by MSE.
 The deposition of the GaN layers may be performed in a dual mode MBE/MSE
 system. The MSE technique differs from conventional MBE in that an
 ultrahigh vacuum dc magnetron sputter cathode is used as the group III
 source and deposition of the layers occurs in the pressure range of 1-5
 mTorr.
 Typically, the MSE technique is employed only for the growth of the
 buffer/nucleation layer. The GaN layer is deposited by ammonia MBE where a
 conventional dual filament K cell is used for the gallium source, and high
 purity ammonia is used as the source of nitrogen.
 Preferably, the deposition system is equipped with a substrate holder
 capable of heating the 2 in. sapphire(0001) wafers to temperatures in
 excess of 1000.degree. C. Typical growth temperatures for the GaN layers
 were in the range of 860-920.degree. C. as measured by an optical
 pyrometer (emissivity set to 0.3).

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The dual MBE (molecular beam epitaxy)/MSE system shown in FIG. 1 comprises
 a vacuum chamber 10 with a nitrogen cryoshroud 12, and port(s) 14 for
 K-cells, port 16 for a 3-source alkyl/gas injection cell, and port(s) 18
 for modified-UHV magnetron sputter sources, such as aluminum. Cryopump 20
 is connected to the chamber 10.
 The main chamber 10 is cryo/turbopumped to a base pressure of
 &lt;5.times.10.sup.-11 torr. Substrates up to 4" diameter are introduced into
 the main chamber via a load-lock and transfer arm. A BN coated graphite
 heater is used to heat substrates up to 1200.degree. C. Substrates are
 typically 2" (0001)sapphire coated on the back surface with molybdenum.
 Smaller pieces are mounted using indium solder to a 2" silicon wafer.
 With this system, a solid MBE-grade aluminum source can be D.C. sputtered
 using an argon plasma operating at 1.about.3 mtorr [target power of 50
 Watts, 400 Volts]. High purity ammonia introduced via the gas injection
 cell is used as the nitrogen source [flow of 15 sccm]. The magnetron
 plasma is sufficient to provide a source of nitrogen ions for the growth
 of AlN. Growth rates for the MSE grown AlN buffer layer at 880.degree. are
 typically 0.15 to 0.25 .mu.m/hr.
 EXAMPLE
 Sapphire wafers (backside sputter coated with Mo) were degreased in
 chloroform vapor followed by a 1 min dip in 10% HF, then rinsed in
 deionized water and blown dry with nitrogen Gas. The substrate was then
 introduced into the system load-lock where degassing of the wafer was
 carried out before introduction into the growth chamber 10. The growth
 chamber was fully cryoshrouded with a base pressure of &lt;10-9 Torr. The
 substrate was then heated to a temperature of 1000.degree. C. under 130
 sccm of ammonia for 10 min before cooling to the buffer layer growth
 temperature of 880.degree. C.
 The buffer or nucleation layer of AlN was used. This layer was deposited by
 MSE using a high purity Al magnetron sputter cathode and ammonia. The
 growth of the nucleation and epilayer was monitored using in situ laser
 reflectance spectrometry. A 200 A nucleation layer was deposited at a
 growth rate of 34 A/min with argon and ammonia flows of 40 and 15 sccm,
 respectively. This resulted in a deposition pressure of about 1.4 mTorr.
 Following the deposition of the AlN nucleation layer, the substrate was
 then heated to the GaN epilayer growth temperature. The Ga K-cell
 temperature was adjusted to give a growth rate of from 1-2 .mu.m/h at an
 ammonia flow rate of 50 sccm. Typical base pressures of 3.times.10.sup.-6
 Torr were observed during growth. Both undoped and silicon doped layers
 were grown. For the doped layers, silane was used as the dopant source.
 The formed GaN layers had thicknesses of .about.2 .mu.m.
 The epilayers were characterized using a triple axis x-ray diffractometer
 .omega. and .omega.-2.theta. scans were carried out to determine both the
 mosaicity and crystalline quality of the deposited layers. Hall effect
 mobilities and carrier densities were measured using a van der Pauw
 geometry (sample size of .about.0.5.times.0.5 cm, applied field of 3 kG)
 with soldered indium dots as the ohmic contacts. The contacts were
 verified to be ohmic by I-V measurements. Photoluminescence (PL) was
 performed at room temperature (RT) and 4 K using excitation from a He--Cd
 laser with an incident power density of &lt;0.25 W/cm2.
 Table 1 gives the observed x-ray linewidths and electrical data for a
 number of layers grown at several different temperatures. All data shown
 in Table I are for layers that were intentionally doped to
 1-7.times.10.sup.17 cm.sup.-3. As shown in Table 1, the minimum linewidths
 for the .omega. and .omega.-2.theta. scans were 210 and 13.7 arcsec,
 respectively, with the highest observed mobility of 560 cm.sup.2 /V s for
 a carrier density of 1.44.times.10.sup.17 cm.sup.-3.
 TABLE 1
 X-ray X-ray Electron
 Layer .omega. scan .omega. 2.theta. scan Carrier density mobility
 No. (arcsec) (arcsec) .eta. (.times. 10.sup.17 cm.sup.-3) .mu.
 (cm.sup.2 /Vs)
 1 210 13.7 X.5 356
 2 271 17.0 7.0 400
 3 299 14.8 7.0 340
 4 377 15.8 3.5 407
 5 303 14.2 1.4 560
 6 324 14.5 1.6 547
 7 323 14.9 1.3 544
 FIG. 2 gives a plot of the observed PL spectrum for a representative layer
 of GaN. As shown in FIG. 2, even at very low excitation intensities the
 yellow luminescence band is very weak at room temperature and virtually
 absent at 4.degree. K. The spectra are dominated by strong donor-bound
 exciton emissions at 3.48 and 3.42 eV with FWHM of 4.9 and 47 meV for
 temperatures of 4 K and room temperature, respectively. At 4 K only very
 weak emission from donor-acceptor transitions are observed, which would
 indicate a low density of defects and/or impurities. This is consistent
 with the observed high mobilities and correspondingly low compensation for
 these layers.
 FIG. 3 shows the measured mobility and carrier density of a representative
 GaN layer. As shown in FIG. 3, a peak mobility of 952 cm2/V s is observed
 at a temperature of 145 K. The corresponding carrier density shows a
 single activation energy of about 13 meV, which is similar to that
 observed previously for silicon doped GaN.
 By the described technique, MBE, GaN layers with electron mobilities of up
 to 560 cm2/V s can be successfully grown with good reproducibility. This
 indicates that using the MBE technique with an MSE grown AlN buffer layer,
 the type of defects affecting the electron mobility can be reduced
 significantly as they have been using MOVPE but without the attendant
 disadvantages of MOVPE.