Solar-blind ultraviolet detectors (SBUVDs) are of large research and technological interest due to their potential in a pallet of applications such as flame sensors, space communications, control systems, ozone holes localization, navigation, etc. ^[1]. Gallium oxide (Ga₂O₃) is a very promising semiconductor for the development of SBUVDs due to its ultra-wide band gap energy value of E_g = 4.4–5.5 eV, and excellent stability in harsh environments especially under radiation^[1−7]. Ga₂O₃ has several polymorphs. The thermodynamically stable one is the monoclinic β-Ga₂O₃ polymorph, which has been the most studied within the last decade^[8]. Several polymorphs of Ga₂O₃ have been identified such as: α,δ, γ and κ(ε)^{[3, 9−11]}.

The metastable κ(ε)-Ga₂O₃ polymorph has E_g = 4.4–4.9 eV and the thermal stability up to 700 ℃, that make possible to compete with the β-Ga₂O₃ polymorph in certain applications^{[12, 13]}. The prospects of κ(ε)-Ga₂O₃ for the development of Schottky barrier and p−n heterojunction diodes^[14−16], a high electron mobility transistors^[17−20], a gas sensors^{[21, 22]}, a X-ray detectors^[23] and SBUVDs^[24−34] were demonstrated already. Moreover, the κ(ε)-Ga₂O₃ crystal belongs to the hexagonal syngony and therefore, it has a good crystallographic matching with sapphire and Ⅲ-nitride crystals, which is of interest for the development of novel optoelectronic devices like e.g., matrix photodetectors^[35]. These devices require a high uniformity of the active layer of the heterostructure^{[34, 36, 37]}. The κ(ε)-Ga₂O₃ can satisfy this requirement due to its low concentrations of defects at the interface and high crystalline symmetry compared with the low-symmetry monoclinic β-Ga₂O₃^[4].

Halide vapor phase epitaxy (HVPE) is a well-established inorganic chemical vapour deposition method for heteroepitaxial growth of GaN, which recently has been employed to grow different Ga₂O₃ polymorphs of high quality, e.g., the metastable α-Ga₂O₃ and κ(ε)-Ga₂O₃ polymorphs^{[20, 38, 39−43]}. The metal−semiconductor−metal (MSM) structures based on HVPE deposited α-Ga₂O₃ demonstrated high response to shortwave ultraviolet radiation^[44−46]. HVPE allows to deposit high quality metastable α-Ga₂O₃ and κ(ε)-Ga₂O₃ polymorphs layers with thicknesses in the wide interval. This method is characterized by a high deposition rate combined with uniform n-type doping^[38]. Nevertheless, of the advantages discussed above, the photoelectric properties of HVPE deposited κ(ε)-Ga₂O₃ are practically unexplored. That is why, the goal of our research is to gain insight into the photoelectric properties of MSM-structures based on the HVPE deposited κ(ε)-Ga₂O₃ layers.

The innovative nature of our work is the research and development of self-powered SBUVDs based on HVPE-κ(ε)-Ga₂O₃ layers with the highest operation rate among same devices based on κ(ε)-Ga₂O₃. Such devices have a number of advantages: not required external power; high-speed performance; stability of characteristics^[5]. In addition, the operating spectral range of the detectors demonstrated lays at a wavelength shorter than 280 nm, and the devices do not require utilization of optical filters.

The κ(ε)-Ga₂O₃ layer deposition was multi-staged. At the first step, a semi-insulating (SI) GaN layer was deposited by the industry-relevant metal organic chemical vapor phase deposition, providing deposition of semiconductors of high quality and homogeneity on large areas^[47−49], on the c-plane patterned sapphire substrates (PSS). The SI-GaN was a template for the κ(ε)-Ga₂O₃ layer. At the second step, a κ(ε)-Ga₂O₃ layer was deposited on the SI-GaN by the HVPE, employing a hot-wall home-made reactor providing high thickness and properties homogeneity on large areas^{[11, 20, 22]}. Vapor phase GaCl and O₂ were utilized as precursors for elemental Ga and oxygen. GaCl vapors was produced by the reaction of metallic Ga and gaseous HCl. The molar flow ratio of the Ⅴ/Ⅲ components was 4.2. Argon was used as the carrier gas. The total flow of Ar was kept at 10 slm. The optimal HVPE deposition temperature for formation of the κ(ε)-Ga₂O₃ polymorph was determined as 630 ℃. The deposition rate during HVPE was ~1.56 µm/h. Intentional doping of the κ(ε)-Ga₂O₃ films during the HVPE growth process was not performed.

Pt interdigital contacts were formed on the κ(ε)-Ga₂O₃ layers by means of magnetron sputtering with following photolithography. The interelectrode distance was 300 µm. The device wafer prepared was divided into chips with 5 mm × 5 mm in size (Fig. 1). The effective area S of the samples surface exposed to irradiation was 7.6 mm².

The phase composition and degree of crystallinity of the samples were examined by X-ray diffraction (XRD) by means of a DRON 6 diffractometer (Bourevestnik, JSC) with CuK_α radiation at wavelength λ = 1.5406 Å. The cross-sectional images of the samples were studied by a scanning electron microscope (SEM) (Phenom ProX, The Netherlands) at accelerating voltage of 10 kV. The transmission spectra of the κ(ε)-Ga₂O₃ layers were calculated from the reflection spectra initially measured by means of a ultraviolet−visible (UV−VIS) spectrophotometer (Analytik Jena, Germany) in the interval of λ = 230–360 nm.

The spectral dependencies of the photoelectric properties were measured by means of the spectrometric system based on a MonoScan 2000 monochromator (Ocean Optics) and a DH-2000 Micropack lamp, described in detail in Ref. [46]. A krypton-fluorine lamp VL-6.C was used as the source of irradiation at λ = 254 nm and the light power density P = 620 μW/cm². The photoelectric and electrical properties of the samples were measured by means of Keithley 2636 A.

A typical XRD spectrum of the structure is illustrated in Fig. 2(a). The XRD peaks at 2θ = 18.7°, 38.6°, 59.5°, 83.1°, and 112.1° correspond to (002), (004), (006), (008), and (0010) Bragg reflections of the κ(ε)-Ga₂O₃^{[50, 51]}, respectively. The other high intensity peaks appearing at 2θ = 41.4°, 90.5° and 34.3°, 72.6° belong to (0006) and (00012) reflections of the Al₂O₃ substrate, (002) and (004) reflections of the GaN layer, respectively. Obviously, we have achieved heteroepitaxial growth of the κ(ε)-polymorph of Ga₂O₃ on GaN buffered sapphire.

Transmittance T of the samples in the interval of λ = 300–360 nm is measured to be within the interval 63%–66% (depicted in Fig. 2(b)). Obviously, the sharp drop of T with decrease of λ from 285 to 260 nm is caused by the interband photons absorption. There are no impurity tails in the T(λ) dependence. The E_g of the samples is 4.72 ± 0.05 eV and corresponds to the literature data^{[19, 25, 28, 29, 34]}. In Fig. 2(b), α is the absorption coefficient.

According to the SEM study (Fig. 2(c)), the thicknesses of the κ(ε)-Ga₂O₃ and GaN layers are ~13.1 µm and ~4.5 µm, respectively. A clear boundary contrast is observed at the interfaces of κ(ε)-Ga₂O₃/GaN and GaN/PSS.

The current–voltage I–V curves of the samples are approximated by power function I ~ U^l (Fig. 3), where U is the applied voltage; l is the power index, l = 3.32 ± 0.03 and 2.54 ± 0.01 in dark condition and under radiation exposure, respectively. The I–V curves are typical for MSM structures with a Schottky barrier at metal/semiconductor interfaces. The dark current I_D rises with U more significant than the total current I_L. I_D increases from 0 to 43.5 µA and I_L increases from 55.3 nA to 197 μA with increase of U from 0 to 3 V. The noticeable conductivity of the HVPE κ(ε)-Ga₂O₃ layers in dark conditions is due to the presence of intrinsic electrically active defects of donor-type. Such defects in the HVPE κ(ε)-Ga₂O₃ layers were investigated by our group in detail in Ref. [20], however, their nature has not been definitively established yet.

Dependencies of the photo to dark current ratio (PDCR), responsivity R, detectivity D, and external quantum efficiency (EQE) of samples on λ and U were computed by means of following formulas, consequently:

1PDCR=Iph/ID,

2R=Iph/(P×S),

3D=R×S1/2/(2×q×ID)1/2,

4EQE=[R×h×c/(q×λ)]×100%,

where I_ph is the photocurrent, I_ph = I_L – I_D; h is the Planck constant, c is the speed of light in vacuum; q is the electron charge. Eq. (3) allows to estimate D in case of the SBUVD noise related with the noise of I_D^[2]. This formula does not take into account other types of noise that occur during the manifestation of the photoresponse enhancement. However, it is often used to evaluate D in this case^{[1, 2, 27, 29]}.

The samples demonstrate weak sensitivity to irradiation at λ ≥ 280 nm (Fig. 4). A slight increase of the PDCR, R, D and EQE in the interval of λ = 280–340 nm is due to the presence of GaN template. The sharp increase in PDCR, R, D and EQE with a drop of λ from 260 to 200 nm is caused by the interband photons absorption in the κ(ε)-Ga₂O₃. The maximum values of the photoelectric properties are observed at λ = 200 nm. The PDCR, R, EQE and D at λ = 200 nm and U = 1 V are determined to have the following values: 180.86 arb. un., 3.57 A/W, 2193.6% and 1.78 × 10¹² Hz^0.5∙cm∙W⁻¹, respectively.

Fig. 5 shows the dependencies of photoelectric properties of the samples studied on U. These dependencies are determined by the I–V curves in dark conditions and under radiation exposure and are approximated by power function with power indexes of –0.96 ± 0.05, 2.36 ± 0.01, 0.58 ± 0.02 for PDCR, R and EQE, D, respectively. The R, EQE and D significantly increase with an increase in U from 0.1 to 3 V. The R, EQE and D at λ = 254 nm, P = 620 µW/cm² and U = 3 V have been determined as: 2.60 A/W, 1272.9% and 1.92 × 10¹¹ Hz^0.5∙cm∙W⁻¹, respectively. On the contrary, the PDCR is significantly reduced with an increase in U from 0.1 to 3 V (Fig. 5(a)) due to rise of the dark current (Fig. 3). The PDCR at U = 3 V is 3.54 arb. un. More interesting is the fact that the I_ph at λ = 254 nm, P = 620 µW/cm² and U = 0 was 55.3 nA. Obviously, the samples are applicable for the self-powered operation mode. The R and EQE in the self-powered operation mode (at U = 0) have 0.9 mA/W and 0.46%, respectively.

The operation rate of the SBUVDs was studied in the self-powered operation mode. The rise τ_r and the decay τ_d times were determined as the time interval between 0.9 and 0.1 levels of the maximum I_L (Fig. 6). τ_r and τ_d do not exceed 100 ms. The self-powered HVPE κ(ε)-Ga₂O₃ SBUVD are characterized by fast photoresponse. It is worth noting that the equipment used to measure the photoelectric properties does not allow to measure the τ_r and τ_d at <100 ms with a high accuracy.

The existence of a built-in electric field at the Pt/κ(ε)-Ga₂O₃ interface gives the capability of the HVPE κ(ε)-Ga₂O₃-based SBUVDs to functionalize in the self-powered operation mode. The self-powered operation mode of SBUVDs was discussed in detail in Refs. [52−54]. The built-in electric field separates the photo-generated free charge carriers that lead to appear an electric current in the circuit.

In Table 1 the photoelectric properties of self-powered SBUVDs based on different Ga₂O₃ polymorphs from literature and our achievements are compared. In Table 1: NWs is the nanowires; a-Ga₂O₃ is the amorphous Ga₂O₃; HJ is the heterojunction; SBD is the Schottky barrier diode. In comparison with the results of other works the samples studied are characterized by low τ_r and τ_d, relatively high values of the R.

SBUVDs based on the HVPE κ(ε)-Ga₂O₃ layers are characterized by high sensitivity to UV radiation, but their operation rate is still low in comparison with commercial devices. The characteristics of the commercial SBUVDs were summarized in our previous work^[46]. Matrix SBUVDs are of interest for many practical applications. In addition, the HVPE growth process of the κ(ε)-Ga₂O₃ layers should be further optimized for the development of simultaneously highly sensitive, high-speed performance and cost-effective SBUVDs. Thus the future research and developments are directed to improve the operation rate of the SBUVDs, to develop matrix detectors based on HVPE κ(ε)-Ga₂O₃ layers as well as to optimize the HVPE process in order to produce cost-effective SBUVDs based on κ(ε)-Ga₂O₃ layers.

Notably, the high values of photoelectric properties exhibited in Fig. 4 and Fig. 5 indicate the manifestation of an enhancement of photoresponse. We had been reporting that self-localization of holes is the main mechanism of an enhancement of the photoresponse for the MSM structure based on the HVPE deposited α-Ga₂O₃ layer with ohmic Ni/Ti contacts^{[46, 65]}. Therefore, reducing of the potential barrier at metal/semiconductor interfaces is also feasible mechanism of an enhancement of the photoresponse for the MSM structure composed of the HVPE deposited κ(ε)-Ga₂O₃ layer with Pt contacts. The current flowing through the Schottky barrier is described by following expressions^[29]:

5I=Is×exp[(eU/(nkTK)−1],

6Is=AT2×exp[−eφb/(kTK)],

where I_s is the saturation current; n is the ideality factor; k is the Boltzmann constant; T_K is the absolute temperature; A is the Richardson constant; φ_b is the Schottky barrier. The φ_b value can be compute from Eqs. (5) and (6)^{[66, 67]}:

7φb=[eU/(kT)]×ln[AT2/Is].

The assessments by means of Eqs. (5)–(7) showed that the φ_b values were 1.05 and 0.89 eV in dark conditions and under radiation exposure at λ = 254 nm and P = 620 µW/cm². The φ_b values in dark conditions correspond to literature data^[68]. A decrease in φ_b values by 0.16 eV under irradiation occurs due to the accumulation of self-trapped holes near the electrode’s regions^[69−71].

We have developed and demonstrated the advantages of shortwave ultraviolet radiation detectors based on the κ(ε)-Ga₂O₃ layers with Pt interdigital contacts. 13.1 μm thick κ(ε)-Ga₂O₃ layers were deposited by the halide vapor phase epitaxy on the patterned sapphire substrates with a GaN template. The spectral dependencies of the photoelectric properties of structures were analyzed in the wavelength interval of 200–370 nm. The detectors based on HVPE deposited κ(ε)-Ga₂O₃ demonstrated a significant response to shortwave ultraviolet radiation with a wavelength no more than 260 nm. The maximum photo to dark current ratio, responsivity, detectivity and external quantum efficiency of structures were 180.86 arb. un., 3.57 A/W, 1.78 × 10¹² Hz^0.5∙cm∙W⁻¹ and 2193.6%, respectively, at a wavelength of 200 nm and applied voltage of 1 V. The high values of the photoelectric properties were caused by the manifestation of an enhancement of the photoresponse mainly due to a decrease in the Schottky barrier at the Pt/κ(ε)-Ga₂O₃ interface under ultraviolet radiation exposure. The detectors had sensitivity to shortwave ultraviolet radiation at zero applied voltage and could functionalize in self-powered operation mode due to built-in electric field at the Pt/κ(ε)-Ga₂O₃ interfaces. The responsivity and external quantum efficiency of structures at a wavelength of 254 nm and zero applied voltage were 0.9 mA/W and 0.46%, respectively. The rise and decay times in self-powered operation mode do not exceed 100 ms, i.e., detectors developed are fast responding.