Diffusion length of non-equilibrium minority charge carriers in β-Ga2O3 measured by electron beam induced current

Abstract

The spatial distribution of electron-hole pair generation in b-Ga 2 O 3 as a function of scanning electron microscope (SEM) beam energy has been calculated by a Monte Carlo method. This spatial distribution is then used to obtain the diffusion length of charge carriers in high-quality epitaxial Ga 2 O 3 films from the dependence of the electron beam induced current (EBIC) collection efficiency on the accelerating voltage of a SEM. The experimental results show, contrary to earlier theory, that holes are mobile in b-Ga 2 O 3 and to a large extent determine the diffusion length of charge carriers. Diffusion lengths in the range 350-400 nm are determined for the as-grown Ga 2 O 3 , while processes like exposing the samples to proton irradiation essentially halve this value, showing the role of point defects in controlling minority carrier transport. The pitfalls related to using other popular EBIC-based methods assuming a point-like excitation function are demonstrated. Since the point defect type and the concentration in currently available Ga 2 O 3 are dependent on the growth method and the doping concentration, accurate methods of diffusion length determination are critical to obtain quantitative comparisons of material quality.

Hydrogen plasma treatment of β -Ga2O3: Changes in electrical properties and deep trap spectra

“…After annealing at 450 C, the diffusion length was 70 nm, and after 550 C, it was 80 nm. The donor concentration and the E2 Ã and E3 trap concentrations in the reference sample were an order of magnitude higher than in HVPE films studied by us previously, 20,24,25 which correlates with the lower L d in the reference sample compared to previously reported for HVPE b-Ga 2 O 3 [L d $ 450 nm (Refs. 20, 24, and 25)].…”

“…Characterization consisted of capacitance vs voltage, C-f measurements at frequencies from 20 Hz to 1 MHz, capacitance-voltage (C-V) profiling at different frequencies, admittance spectra (AS), 17 deep level transient spectroscopy (DLTS), 18 current DLTS (CDLTS), 18 and photoinduced current transient spectroscopy (PICTS) 19 in the temperature range of 77-500 K. Diffusion length L d of nonequilibrium charge carriers was measured at 300 K by monitoring electron beam induced current (EBIC) collection efficiency on the probing electron beam energy of the scanning electron microscope (SEM). [20][21][22] Before H plasma exposure, the reference sample capacitance at 300 K was 650 pF, independent of frequency ( Fig. 1), and C-V profiling up to À5 V yielded the net shallow donor concentration of N d ¼ 2 Â 10 17 cm À3 .…”

Defects responsible for charge carrier removal and correlation with deep level introduction in irradiated β-Ga2O3

“…[16][17][18][19][20][21] Theory also points to the absence of impurities suitable as shallow acceptor dopants and the role of polaronic states of self-trapped holes (STHs) that result in low hole mobility even when nonequilibrium holes are created by illumination. 17,22 Studies using the Hall effect, 23,24 deep level transient spectroscopy (DLTS), [23][24][25][26][27][28] deep level optical spectroscopy (DLOS), 23,27 admittance spectroscopy (AS), 29,30 light capacitance voltage (LCV) profiling, photocapacitance spectroscopy (PC), 23,[28][29][30] photoluminescence (PL) and micro-cathodoluminescence (MCL) spectroscopy, 31,32 localized vibrational mode (LVM) spectroscopy, 33 positron annihilation (PA), 34 and electron beam induced current (EBIC) 28,35 have established the positions of major electron traps in the upper half and deep acceptors in the lower half of the bandgap and compensation by gallium vacancy acceptors or their complexes with shallow donors in as-grown or irradiated films and crystals and the energy level positions of transition metal impurities such as Fe. 23,26 EBIC and DLTS measurements with optical excitation (ODLTS) demonstrate that nonequilibrium holes are mobile in b-Ga 2 O 3 at moderate temperatures, with an activation energy of transition from polaronic STH states to valence band holes being lower than predicted.…”

“…23,26 EBIC and DLTS measurements with optical excitation (ODLTS) demonstrate that nonequilibrium holes are mobile in b-Ga 2 O 3 at moderate temperatures, with an activation energy of transition from polaronic STH states to valence band holes being lower than predicted. 17,22,28,30,35 A better understanding of the defects in b-Ga 2 O 3 is necessary to assess the role this material will play relative to the established wide-bandgap materials SiC and GaN in further evolution of high-power electronics and optoelectronics. In this paper, we report measurements of electrical and recombination properties and deep trap spectra in b-Ga 2 O 3 irradiated with protons and a-particles to clarify the role of deep traps in compensation of conductivity and in recombination processes.…”

Microscopy of defects in semiconductors

“…Since the dark spots in the CL image appear in the same location of the corresponding EBIC image, they are not caused by carrier escape from the junction but rather non-radiative recombination at defects, possibly clusters of point defects in the MQW region. Other examples for EBIC include the investigation of grain boundaries in silicon [71], stacking faults in 4H-SiC [75], measurement of the diffusion length in GaN [76,77] and -Ga2O3 [78], recombination behaviour of dislocations in bulk GaN [79] and solar cells [80].…”