Excitonic Recombination Processes in Undoped and Doped Wurtzite GaN Films Deposited on Sapphire Substrates
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at 3.4779 eV which we attribute to the recombination of excitons bound to neutral Si donors. We have performed detailed studies of the excitation power and temperature dependence of the PL spectra to identify the features associated with excitons bound to native neutral donors and free excitons. In addition, we have estimated the binding energy of the exciton to a neutral native donor and to the neutral Si donor. EXPERIMENT The growth of undoped and silicon-doped (Si-doped) GaN films were done in an inductively-heated, water cooled, vertical reactor. The source gases, electronic grade ammonia (NH 3) and ultra-high purity trimethylgallium (TMGa), were introduced separately via pressurecontrolled manifolds into the top of the reactor, which was held at 57 torr. A nucleation layer was formed at 450 'C onto a sapphire substrate before the film deposition. The film growth was carried out at substrate temperatures between 1000-1080 'C. Under usual growth condition the described reactor yielded a growth rate of 1.1 jim/hr [5]. Si-doping of GaN films were achieved by using Si 2H6 dilute in either H2 or in N 2. In some instances the doped GaN was deposited on a high resistivity 0.5 gtm thick unintentionally-doped GaN layer. The doping level was found to be controllable over the range from lxl10 7 cm 3 to 4x10' 9 cm-3 . Comparison between SIMS and room temperature (RT) Hall data suggested a nearunity activation efficiency of Si in the GaN lattice [3]. High frequency FETs have been successfully fabricated on films doped using this procedure [2]. The low temperature PL experiment was carried out with samples placed in a continuous flow liquid helium cryostat and the temperature was kept constant at 6K. The variable temperature measurements were performed in the temperature range between 5K to 320K using a needle valve to control the amount of liquid helium allowed in the sample chamber and the heat generated on resistors placed in the helium diffuser and sample holder. The samples were excited with the 325.0 nm (3.81 eV) line from a He-Cd laser. The laser excitation intensity was controlled with UV transmitting neutral density filters. The radiation emitted by the samples was dispersed and analyzed by a double spectrometer fitted with a 1800 gr/mm grating set and detected with a UV-sensitive GaAs photomultiplier operated in photon counting mode. Different spectrometer band passes were used for diverse spectral ranges. Although the PL spectra presented here are not corrected for instrumental response, we have corrected the spectrometer wavelength calibration near the bandedge emission bands. RESULTS AND DISCUSSION We have reported previously the LTPL spectra of nominally undoped GaN films, which commonly have electron concentrations between 5x10 16 and 5x10 17 cm-3, and electron mobilities between 250 and 400 cm 2/V-s [5,6]. The PL spectra of such films are characterized by a dominant sharp emission line about 3.475 eV and a broad band which peaks around 2.25 eV. The former is assigned to excitons bound to a neutral unknown do
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