Cathodoluminescence studies of electron injection effects in p-type gallium oxide Open Access

Electron injection plays a critical role in manipulating minority carrier transport within semiconductors, particularly in ultra-wide-bandgap materials, such as gallium oxide, which is becoming increasingly important for high-power and advanced optoelectronic devices.^1–6 In a semiconductor, the majority carriers are the charge carriers that naturally dominate conduction (electrons in n-type and holes in p-type). Minority carriers, on the other hand, are the opposite type and exist in much lower concentrations. However, their transport properties are crucial for the functionality of bipolar devices.

One of the most significant effects of electron injection is its ability to extend the minority carrier diffusion length.⁷ This length represents the average distance a minority carrier can travel before recombining with a majority carrier. This recombination process effectively eliminates the minority carrier from contributing to conduction.

Electron injection can positively impact diffusion length by “passivating” recombination centers.⁷ These centers are microscopic defects within the semiconductor lattice that act as traps for majority and minority carriers. By filling these traps, electron injection essentially reduces the availability of recombination sites. Consequently, minority carriers have a higher probability of surviving longer and traveling further before encountering a recombination event, leading to an increased diffusion length.

The impact of electron injection on minority carrier diffusion length, due to irradiation with 10–20 keV electron beam of a scanning electron microscope (SEM), has recently been studied in n- and highly resistive p-type β-Ga₂O₃/c-sapphire films with (−201) preferential orientation.^8,9 The effect was ascribed to trapping of non-equilibrium electrons,¹⁰ generated by the primary SEM electron beam, at native defects-associated metastable levels in the bandgap of Ga₂O₃. In this work, the increase in minority carrier diffusion length in homoepitaxial p-type (010)-oriented Ga₂O₃ layers was studied under electron beam irradiation and was complemented with cathodoluminescence emission from the same region, providing insight into the mechanism of the phenomenon.

In contrast to previous reports^8,9 on studies of electron injection effects in Ga₂O₃, in which Electron Beam-Induced Current (EBIC) technique in situ in SEM was predominantly used, this work focused on variable temperature cathodoluminescence (CL) studies, thus shedding light on the lifetime of non-equilibrium carriers. In addition, in the present work, the two independent techniques—EBIC and CL—were correlated, to better understand the relationship between the minority carrier diffusion length and lifetime.

Undoped 1 μm-thick β-Ga₂O₃ was grown on (010)-oriented insulating Fe-doped Ga₂O₃ in a RF-heated horizontal metalorganic chemical vapor deposition (MOCVD) reactor using Ga/O ratio and growth temperature of 1.4 × 10⁻⁴ and 775 °C, respectively.^11,12 X-ray diffraction revealed high quality layer of β-Ga₂O₃ with monoclinic space group (C2/m) symmetry.

Metal contacts for electrical characterization were prepared by Ti/Au deposited at the four corners of the sample in a van der Pauw configuration. The contacts were tested by measuring I–V characteristics, which showed the Ohmic dependence in the temperature range of 450–850 K. Because the contacts exhibited deviation from the linear I–V dependence below 450 K, the Hall effect measurements were not conducted at room temperature. The positive Hall voltage increased with increasing magnetic field, thus confirming the p-type nature of the epitaxial layer with hole concentration p ∼ 2 × 10¹⁷ cm⁻³ and resistivity ρ ∼ 0.39 Ω·cm at 450 K. Detailed studies of electrical properties for the epitaxial layer under test will be outlined in a separate article under preparation.

Minority carrier diffusion length, L, measurements were carried out using electron beam-induced current technique in situ in Phillips XL-30 SEM using planar line-scan electron beam excitation with an electron beam moving along the sample’s surface.^7,10,13,14 The EBIC measurements were carried out at room temperature under an electron beam accelerating voltage of 20 kV (to cover the whole epitaxial layer thickness), corresponding to ∼0.6 nA absorbed current (measured with Keithley 480 picoammeter) and ∼1 µm electron range (penetration depth) in the material.¹⁴ The EBIC line-scans (16.3 µm lateral length) for diffusion length extraction were carried out using Ni/Au (20/80 nm) asymmetrical pseudo-Schottky contacts created on the film with lithography/liftoff techniques.

Here, I(x) is the electron beam-induced current signal as a function of coordinate; I₀ is a scaling factor; x is the coordinate measured from the edge of the contact (Ni/Au) stack; and α is a recombination coefficient (set at −0.5).

To perform electron injection in the region of EBIC measurements, line-scans were not interrupted for the total time of up to ∼1200 s (corresponding to the primary excitation electron charge density of 3.2 × 10⁻⁷ C/μm³). The values of diffusion length were periodically extracted using Eq. (1) for different incremental durations of electron injection varying from nearly zero (for the first line-scan) to 1200 s. EBIC signal was amplified with Stanford Research Systems SR 570 low-noise current amplifier and digitized with Keithley DMM 2000, controlled by a PC using home-made software.

It should be noted that the primary excitation SEM electron beam serves for generation of non-equilibrium electron–hole pairs in the material due to the band-to-band (valence band to conduction band) transition of excited electrons. The primary excitation electrons do not accumulate in the material since the sample is grounded, thus preserving the sample’s electroneutrality.

Cathodoluminescence (CL) measurements were carried out in the 300–330 K temperature range at 20 kV accelerating voltage using Gatan MonoCL2 attachment to the SEM integrated with a variable temperature stage and an external controller. Spectra were recorded with a Hamamatsu photomultiplier tube sensitive in the 150–850 nm range and a single grating monochromator (blazed at 1200 lines/mm).¹⁵ 

Figure 1 demonstrates a series of room temperature continuous CL spectra from the 10 µm² area on the surface of gallium oxide as a function of electron beam irradiation duration (see above for the electron beam injection regimes). The observed CL spectra are typical for Ga₂O₃, and instead of showing near band edge emission, they consist of several broad emission bands. The origins of broad emission bands in β-Ga₂O₃ include complexes of O and Ga vacancies, O-related complexes, and O and Ga interstitials. Detailed studies of n- and p-type Ga₂O₃ optical properties have recently been presented in Refs. 13 and 16. It could be seen from the spectra in Fig. 1 that the CL intensity shows a continuous decrease with electron irradiation duration ranging from nearly zero (less than 20 s time difference; spectrum 1) to 1440 s (spectrum 4).

The peak intensity for each spectrum, presented in Fig. 1 at room temperature, is correlated in Fig. 2 with the minority carrier diffusion length, measured as a function of electron beam irradiation duration in the same area of Ga₂O₃. In agreement with previous studies on n- and highly resistive heteroepitaxially grown p-type Ga₂O₃,^8,9 minority carrier diffusion length in Fig. 2 exhibits a linear increase (before saturation) consistent with a decrease in peak CL intensity. The mechanism responsible for the effects in Fig. 2 is detailed in Refs. 8, 9, 16, and 17 and is ascribed to trapping of non-equilibrium carriers, generated by the primary electron beam, at native defect levels in the forbidden gap.

References 19 and 20 summarize traps in Ga₂O₃, which are associated with native defects and impurities. According to Ref. 19, gallium vacancy (V_Ga)-related energetic levels are located at 0.1–0.3 and 0.3–0.5 eV above the top of the valence band. In a previous study focused on electron beam irradiation impact on minority carrier diffusion length in highly resistive p-type Ga₂O₃/c-sapphire,⁹ the activation energy around 0.1 eV was identified. The relative proximity of the activation energies obtained for the different samples and from the independent experimental techniques—EBIC in Ref. 9 and CL in this work—suggests involvement of the same defect levels in both cases.

In this work, variable temperature cathodoluminescence studies of undoped p-type gallium oxide epitaxial layer were carried out and correlated with minority carrier diffusion length measurements, using two independent CL and EBIC techniques, in the same region subjected to electron beam irradiation. Elongation of the diffusion length and a simultaneous CL intensity decay (from the same region) with increasing duration of electron beam irradiation were ascribed to charge trapping at the deep metastable levels in the gallium oxide forbidden gap, which leads, in turn, to a longer non-equilibrium minority carrier lifetime in the conduction band. The activation energy of ∼0.3 eV, associated with the impact of electron beam irradiation (injection) on CL emission intensity, was ascribed to gallium vacancy-related point defects. These defects do not necessarily determine the p-type electrical conductivity in the sample but play a significant role in the charge trapping effects. The relative proximity of the activation energies, previously obtained from the EBIC measurements for the highly resistive p-type Ga₂O₃ samples, and the CL measurements, carried out in this work, suggests the similarity of the involved defect levels in both cases.

Finally, the values for minority carrier (electrons) diffusion length obtained in this work (750 nm at nearly zero electron irradiation duration; 12 s difference) are almost a factor of ten larger as compared to the diffusion length (∼100 nm at room temperature) measured for Si-doped n-type gallium oxide epitaxial layers (5 × 10¹⁶–2 × 10¹⁷ cm⁻³ majority electron concentration),^21,22 thus providing additional evidence for a p-type conductivity in the sample under investigation.

The research at UCF was supported in part by NSF (Grant Nos. ECCS2310285, ECCS2341747, and ECCS2427262), US–Israel BSF (Award No. 2022056), and NATO (Award Nos. G6072 and G6194). The work at UF was performed as part of the Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA), sponsored by the Department of the Defense, Defense Threat Reduction Agency, under award HDTRA1-20-2-0002, monitored by Jacob Calkins. The present work is a part of “GALLIA” International Research Project, CNRS, France. GEMaC colleagues acknowledge financial support of French National Agency of Research (ANR), project “GOPOWER,” CE-50 N0015-01. Research at Tel Aviv University was partially supported by US–Israel BSF (Award No. 2022056) and NATO (Award No. G6072).

The authors have no conflicts to disclose.

Leonid Chernyak: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). Alfons Schulte: Funding acquisition (equal); Project administration (equal). Jian-Sian Li: Investigation (equal); Resources (equal). Chao-Ching Chiang: Investigation (equal); Resources (equal). Fan Ren: Funding acquisition (lead); Supervision (lead). Stephen J. Pearton: Funding acquisition (lead); Resources (lead); Writing – original draft (lead); Writing – review & editing (lead). Corinne Sartel: Investigation (equal); Resources (equal). Vincent Sallet: Investigation (equal); Resources (equal); Writing – original draft (equal). Zeyu Chi: Investigation (equal); Resources (equal). Yves Dumont: Funding acquisition (lead); Investigation (lead); Resources (lead); Supervision (lead). Ekaterine Chikoidze: Funding acquisition (lead); Investigation (lead); Resources (lead); Supervision (lead); Writing – original draft (lead). Arie Ruzin: Investigation (equal); Resources (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.