GaN heterojunction

Sensors and Actuators A 232 (2015) 208–213

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Deep ultraviolet photodiodes based on the ␤-Ga2 O3 /GaN heterojunction Shinji Nakagomi ∗ , Taka-aki Sato, Yusuke Takahashi, Yoshihiro Kokubun Department of Information Technology and Electronics, Faculty of Science and Engineering, Ishinomaki Senshu University, Ishinomaki, Miyagi 986-8580, Japan

a r t i c l e

i n f o

Article history: Received 19 March 2015 Received in revised form 25 May 2015 Accepted 14 June 2015 Available online 17 June 2015 Keywords: Gallium oxide Deep UV Photodiode GaN Heterojunction

a b s t r a c t A deep ultraviolet (UV) photodiode was fabricated using a heterojunction between ␤-Ga2 O3 and GaN, and its UV sensitivity was investigated. A thin ␤-Ga2 O3 layer was prepared on p-type GaN template substrate by gallium evaporation in oxygen plasma. The ␤-Ga2 O3 layer had a (−2 0 1)-oriented crystal structure on (0 0 1) GaN. A device based on the ␤-Ga2 O3 /GaN heterojunction exhibited good rectifying properties. Under reverse bias, the current increased linearly with an increase in the deep-UV light intensity. The responsivity of the photodiode was highest under deep-UV light below a wavelength of 240 nm. The response time of the photodiode to deep-UV light was in the order of sub-milliseconds. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Deep ultraviolet (UV) photodetectors have a wide range of applications, including flame sensors, UV radiation monitoring below the ozone hole, and as photodetectors for optical communication in space. ␤-Ga2 O3 , which has a band gap (Eg ) of 4.9 eV, is a promising candidate as a UV photodetector material that is blind to wavelengths above 280 nm, known as a solar-blind photodetector. Kokubun et al. demonstrated photodetection using ␤-Ga2 O3 films prepared on (0 0 0 1) sapphire substrates using the sol–gel method [1]. Oshima et al. demonstrated UV photodetection using single ␤Ga2 O3 crystals [2], and fabricated practical ␤-Ga2 O3 -based flame detectors [3]. Suzuki et al. have also reported the high responsivity for UV photodetection using single ␤-Ga2 O3 crystals and a high resistance cap layer [4]. It is common to use pn junctions for photodetectors. pn junctions are expected to be applied for phototransistor devices and photodiode arrays. However, it is currently difficult to prepare Ga2 O3 pn junctions due to the difficulty in producing p-type Ga2 O3 . One possible solution is to use a heterojunction with another semiconductor in which it is possible to produce p-type conduction.

∗ Corresponding author. Fax:+81 225227746. E-mail address: [email protected] (S. Nakagomi). http://dx.doi.org/10.1016/j.sna.2015.06.011 0924-4247/© 2015 Elsevier B.V. All rights reserved.

In our previous study, we fabricated a deep-UV photodiode using the heterojunction between n-type ␤-Ga2 O3 and p-type 6HSiC (Eg = 3.02 eV) [5]. The deep-UV photodiode was demonstrated with the highest sensitivity to deep-UV light below a wavelength of 260 nm, and the response time to deep-UV light was in the order of milliseconds. In the present study, a deep-UV photodiode was fabricated using a heterojunction between ␤-Ga2 O3 and GaN with a band gap of 3.4 eV, and its UV sensitivity was investigated. It is considered that the combination between ␤-Ga2 O3 and GaN is more promising than that between ␤-Ga2 O3 and SiC. There have been some studies on GaN prepared on ␤-Ga2 O3 single crystals [6,7]. Two crystals of GaN and ␤-Ga2 O3 can be grown with an epitaxial relation to each other, even though GaN has the wurtzite structure and ␤-Ga2 O3 is a monoclinic structure. A UV sensor device with a Ga2 O3 /GaN structure has been developed by oxidation of a GaN thin film on a sapphire substrate [8,9]. However, the ␤-Ga2 O3 layer was not oriented with respect to GaN, so that the heterojunction between Ga2 O3 and GaN could not be used as an active part of the device. We demonstrate here that an oriented ␤-Ga2 O3 thin film can be prepared on a GaN layer. For application as a deep-UV photodiode, the combination of ␤-Ga2 O3 with GaN is expected to reduce the sensitivity for longer wavelengths because GaN has a wider bandgap than SiC (4H- 3.26 eV, 6H- 2.93 eV).

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Fig. 1. XRD pattern for ␤-Ga2 O3 thin film formed on a GaN template substrate on sapphire. XRD intensity is shown on a logarithmic scale.

2. Experimental 2.1. Preparation of ˇ-Ga2 O3 thin films p-Type GaN template substrates with a Mg doping density of 1 × 1019 cm−3 , which was purchased from NTT Advanced Technology Corp., were used. (0 0 1) oriented GaN layers were formed on a buffer layer on the (0 0 1) c-plane of sapphire substrates. The carrier density of the p-type GaN after annealing treatment was estimated to be approximately 1 × 1017 cm−3 . The substrate wafers were cut to a size of approximately 10 × 10 mm2 . Two thin ␤-Ga2 O3 layers with thicknesses of 116 nm and 175 nm were prepared on p-type GaN template substrates by gallium evaporation in oxygen plasma. The substrate temperature was kept at 800 ◦ C and the radio frequency power for the oxygen plasma was 100 W. The method for the formation of ␤-Ga2 O3 layer has been described in reference [10]. The ␤-Ga2 O3 layers had a (−2 0 1)-oriented crystal domain structure. Fig. 1 shows an X-ray diffraction (XRD) pattern ( − 2 scan) for the ␤-Ga2 O3 thin film formed on a (0 0 1) GaN template substrate. Only the (−2 0 1) related peaks of ␤-Ga2 O3 , (0 0 2) peak from GaN, and (0 0 6) peak from sapphire were observed. This indicates that the (−2 0 1) plane of the ␤-Ga2 O3 layer is parallel to both the surfaces of the (0 0 1) GaN layer and the (0 0 1) c-plane of the sapphire substrate. This orientation of ␤-Ga2 O3 crystal is the same as that for ␤-Ga2 O3 layers that were formed directly on (0 0 1) sapphire substrates [10,11].

Fig. 2. Photograph and cross-sectional schematic of the deep UV sensor device based on ␤-Ga2 O3 /GaN heterojunction.

2.3. Measurements Current–voltage (I–V) characteristics of the Deep-UV sensor devices were measured in dark condition and under various UVlight illumination intensities. The relative intensity of the UV-light was increased from 0.1% to 100% using a deuterium lamp and several types of neutral density filters. The light power density of the deuterium lamp was 22 mW/cm2 as a rough estimate using a standard photo diode. The spectral response of the devices was measured in the wide wavelength region from 200 to 500 nm using a monochromator with a xenon arc lamp as the optical excitation source. The transient responses of the photodiodes were measured. The pulses were produced by passing the light from a deuterium lamp through a light chopper. The waveform of the light pulses was monitored using a silicon avalanche photodiode detector (APD) module. 3. Results and discussion 3.1. UV sensing properties

2.2. Device structure Deep-UV sensor devices were fabricated with a planar structure using the ␤-Ga2 O3 layer formed on the GaN template substrate. A cross-sectional schematic diagram of the photodiode structure is shown in Fig. 2. Silicon dioxide was initially formed on the GaN layer by spin-coating of a sol–gel solution as a lift-off layer. The SiO2 layer was etched selectively using the first photolithographic process. The ␤-Ga2 O3 layer was then formed by the evaporation of gallium in oxygen plasma. The SiO2 layer was then etched with HF. Only the ␤-Ga2 O3 layer on GaN was left selectively. The two photolithographic processes were used to obtain layers of Pt/Ti/Pt/Au on the p-type GaN as an ohmic electrode and a thin 10 nm Au layer onto the remaining ␤-Ga2 O3 layer as a semitransparent Schottky electrode. The area of the thin Au electrode was approximately 0.2 mm2 . Fig. 2 shows a photograph of the resultant device. Au wire was connected to the thin Au electrode and ohmic electrode with conductive cement. With the p-type GaN side positive, the forward bias direction was defined for the p–n type device.

Fig. 3 shows the current–voltage (I–V) characteristics of the photodiode in the dark, where Fig. 3(a) and (b) show a semi-logarithm plot and linear plot of the I–V characteristics, respectively. We called the bias forward direction when the p-type GaN was under positive bias. Because the current is increased when the diode is biased in forward direction, we distinguished the diode characteristics are based on p–n heterojunction. The diodes exhibited good rectifying properties. The rectifying ratio was round 1.5 × 105 at 4.5 V. The current increased exponentially following a turn-on voltage of approximately 2.8 V. Fig. 3(a) shows that the forward current increased with a good exponential relationship at bias voltages higher than 3 V and the estimated ideality factor was approximately 3.7. The characteristics in the higher current region indicated a large series resistance of ca. 40 k. It is supposed that the series resistance originates from the p-type GaN and ␤-Ga2 O3 layers. The reverse current was lower than 10−9 A for a reverse-bias voltage up to 8 V under dark conditions. Fig. 3(a) and (b) also show the current–voltage characteristics for the photodiode under various UV-light illumination intensities.

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Fig. 4. Relationship between current response of photodiodes under a reverse bias of 2 V and the relative UV-light intensity for two type diodes with different ␤-Ga2 O3 layer thicknesses.

Fig. 5. Spectral response of photodiodes based on ␤-Ga2 O3 /GaN heterojunction with 116 nm and 175 nm thick ␤-Ga2 O3 layers and based on the ␤-Ga2 O3 /SiC heterojunction with a 116 nm␤-Ga2 O3 layer. Fig. 3. Current–voltage characteristics for various UV-light illumination intensities; (a) semi-logarithmic and (b) linear plots.

The reverse current increased with the UV-light intensity. In addition, the forward current in the range of 1.2–3 V was also increased with the intensity of illumination. Fig. 3(b) suggests that the diode does not behave as an ideal pn junction under light illumination. We think that the photodetection mechanism for the diode includes both a photocurrent in the pn junction and a decrease in resistance of the semiconductor region caused by light illumination. The differential resistance of the diode higher than 1 × 109  in the range between +2 and −6 V was decreased to almost 4 × 107  under UV-light illumination. Fig. 4 shows the relationship between the incident light intensity and the photocurrent at a reverse bias of 2 V for two devices with diodes having different ␤-Ga2 O3 thicknesses of 116 and 175 nm. The photocurrent for both devices increased linearly by three orders of magnitude with increasing UV-light intensity. Linear characteristics were observed for both diodes with different ␤-Ga2 O3 layer thicknesses. The diode with the thinner ␤-Ga2 O3 layer had a higher current level. Noise was apparent at currents lower than 0.5 nA. The spectral response of the devices was measured. The responsivity was calculated from the measured photocurrent divided by the light power of each wavelength. Fig. 5 shows the spectral responsivity on a logarithmic scale of photodiodes based on the

␤-Ga2 O3 /GaN and ␤-Ga2 O3 /SiC heterojunctions. The responses are shown for two diodes with 116 and 175 nm thick ␤-Ga2 O3 layers formed on the p-type GaN layer and a 116 nm thick ␤-Ga2 O3 layer formed on the p-type SiC substrate. The responses of two diodes are shown for each type device. The current responses were measured for the diodes under reverse-bias at 2 V. A responsivity of 0.18 A/W was obtained for the photodiodes based on the ␤-Ga2 O3 /GaN heterojunction at a wavelength of 225 nm. The long wavelength threshold of the spectral response almost corresponds to the band gap energy of ␤-Ga2 O3 , and there is a peak near 375 nm. The wavelength equivalent to the band gap of GaN (3.4 eV) is 365 nm; therefore, the responsivity at 375 nm is due to absorption by the band edge of GaN. The pattern of spectral response was similar to that for a UV sensor device with the Ga2 O3 /GaN structure prepared by the oxidation of GaN, as shown in references [8] and [9]. The spectral responsivity of the photodiode based on the ␤Ga2 O3 /SiC heterojunction with a 116 nm thick ␤-Ga2 O3 layer was also measured [5]. The spectrum had a similar shoulder structure in the region of 350–400 nm. The wavelength equivalent to the band gap of 6H-SiC (3.02 eV) is 410 nm; therefore, this shoulder structure appears to also be caused by absorption in the 6H-SiC substrate region. The responsivities of the photodiodes based on ␤-Ga2 O3 /GaN heterojunction in the region from 250 nm to 350 nm were lower than that of the photodiode based on ␤-Ga2 O3 /SiC

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Fig. 6. (a) Response of photodiode to deep-UV light pulses with a reverse-bias voltage of 2 V. (b) Light waveform measured with a silicon APD module for the same light pulses.

heterojunction. Thus, the spectral selectivity of the response for photodiodes based on the ␤-Ga2 O3 /GaN heterojunction in the deep-UV region is superior to that of the photodiode based on the ␤-Ga2 O3 /SiC heterojunction. Comparison of the responsivity for the photodiodes based on the ␤-Ga2 O3 /GaN heterojunction with 116 nm and 175 nm thick ␤-Ga2 O3 layers revealed that the diode with a thinner ␤-Ga2 O3 layer had higher responsivity. The shoulder structure in the spectral response appeared similarly at 375 nm for the diode with the 175 nm thick ␤-Ga2 O3 layer. The reason is described later. The transient responses of the photodiodes based on the ␤Ga2 O3 /GaN heterojunction were measured. Fig. 6(a) shows the current response of the photodiode to deep-UV light pulses measured with a digital oscilloscope. The diode was under a reverse-bias of 2 V. The photocurrent was calculated from the voltage across a 10 k series resistance connected to the photodiode. The waveform of the light pulses measured with a silicon avalanche photodiode detector (APD) module under the same conditions is shown in Fig. 6(b). The width of the UV-light pulses was ca. 0.5 ms. Although the actual light pulses have an almost rectangular waveform, the photocurrent responses in Fig. 6(a) rose up and fell down with some time constants. The response times were ca. 0.3 ms, which is superior to the response time of 9 ms reported by Oshima et al. for a single crystal ␤-Ga2 O3 UV photodetector [3] and to that of 1.2 ms reported in our previous report for the ␤-Ga2 O3 /SiC heterojunction photodiode [5]. 3.2. Band diagram The expected band diagram for the photodiode under zero bias is shown in Fig. 7(a). n-Type ␤-Ga2 O3 has a wider band gap than p-type GaN, and it has been reported that the electron affinity of both ␤-Ga2 O3 and GaN is approximately 4.0 eV [12,13]. Therefore, the offset in the conduction band EC , is almost zero and the offset in the valence band EV , should be 1.5 eV at the hetero-interface. The resistivity of the prepared ␤-Ga2 O3 layer was experimentally estimated to be ca. 5 M cm using interdigital electrodes. The carrier concentration of the ␤-Ga2 O3 layer was calculated to be ca. 1 × 1010 cm−3 assuming the Hall mobility of 100 cm2 /Vs [14]. The band alignment was roughly estimated using the resistivities of ␤Ga2 O3 and GaN. The energy difference between the Fermi level and the top of the valence band of p-type GaN is estimated to ca. 0.15 eV, while the energy difference between the bottom of the conduction

Fig. 7. Expected band diagram for the ␤-Ga2 O3 /GaN photodiode under (a) zero bias and (b) reverse bias. (c) Schematic light intensity profiles for the case with thick or thin ␤-Ga2 O3 layer.

band and the Fermi level of ␤-Ga2 O3 is 0.55 eV. Both the band offsets and the energy difference between the Fermi levels in ␤-Ga2 O3 and GaN form the heterobarrier of the junction. The barrier height of 2.7 eV for electrons to move from ␤-Ga2 O3 to GaN is considerably lower than the barrier height of 4.2 eV for holes to move from GaN to ␤-Ga2 O3 ; therefore, only the flow of electrons from ␤-Ga2 O3 to GaN can be considered under dark conditions. A Schottky barrier of 1.23 eV is also formed between the Au and ␤-Ga2 O3 layers because the work function of Au is 5.23 eV [12]. The I–V characteristics under dark conditions shown in Fig. 3 reveal a current increase when the ␤-Ga2 O3 /GaN heterojunction was forward-biased at 2.8 V. This voltage is close to the barrier height of 2.7 eV for electrons to move from ␤-Ga2 O3 to GaN. The diode has a structure in which the Schottky junction between the Au and ␤-Ga2 O3 layer and the ␤-Ga2 O3 layer acting as electric resistance is connected with the ␤-Ga2 O3 /GaN heterojunction in the opposite direction; therefore, the Schottky junction is reverse-biased when the diode is biased in the forward direction. The applied voltage is shared among the two junctions and large series resistance of the ␤-Ga2 O3 layer. However, when the

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␤-Ga2 O3 /GaN heterojunction is reverse-biased, the Schottky junction is biased in the forward direction, so that the Schottky junction does little to influence the rectifying properties. Fig. 7(b) shows the expected band diagram of the photodiode under reverse bias. The depletion layer spreads mainly in the ␤Ga2 O3 layer, which has higher resistivity than the GaN layer. In addition, an electric filed is formed in the ␤-Ga2 O3 layer due to the high electric resistance. The majority of the deep-UV light illuminated onto the thin Au electrode seems to be absorbed by the ␤-Ga2 O3 layer. We use a model of the present photo diode composed from three regions of the Schottky junction between Au and ␤-Ga2 O3 , the neutral region of ␤-Ga2 O3 acting as a resistance, and the pn junction between ␤-Ga2 O3 and GaN. The constant applied reverse voltage V is divided to V1 for the Schottky junction, V2 for the resistance region and V3 for the depletion layer of pn junction. V = V1 + V2 + V3 .

(1)

The current of Schottky junction under forward bias is given by I ≈ SA∗ T 2 e−

Bn kT

e

qV1 kT

.

(2)

where S is area of cross section of the diode, A* is effective Richardson constant, T is absolute temperature, ϕBn is Schottky barrier height, k is Boltzmann constant and q is charge of electron. The current in resistance region of ␤-Ga2 O3 layer is given by I=

qS (n20 e + p20 h )V2 . L

(3)

where L is length of neutral region, e and h are mobility of electron and hole, n20 and p20 are concentration of electron and hole in the neutral region of ␤-Ga2 O3 . The current of pn junction under reverse bias is given by I ≈ qS(

De n30 D p20 + h ). Le Lh

an increase of V1 . Also V2 is decreased because V3 is constant for the light illumination. Consequently, a decrease in resistance of ␤-Ga2 O3 layer caused by an increase of photogenerated carrier leads to an increase of forward bias of Schottky junction. This is mechanism of photo detection of the present diode. Light with energy lower than the band gap of ␤-Ga2 O3 permeates through the ␤-Ga2 O3 layer and reaches the GaN layer, where electrons and holes are generated. This corresponds to the significant absorption peak near 375 nm in the spectral responsivity shown in Fig. 5. However, only a small amount of photo-generated carriers in the depletion layer in GaN layer contribute to the photocurrent of the diode because the depletion layer in GaN is thinner than that of the ␤-Ga2 O3 layer. In order to decrease the responsivity of the photodiode for longer wavelengths, it is necessary to make the depletion layer in GaN thinner by doping the GaN surface region, because the carrier concentration of GaN substrate is close to 1 × 1017 cm−3 , even if activation annealing of the dopant in GaN was effective. However, to increase the responsivity of the photodiode for deep UV-light requires optimization of the ␤-Ga2 O3 layer thickness, so that the majority of the deep UV-light is absorbed in the depletion layer of ␤-Ga2 O3 under bias. Fig. 7(c) shows schematic light intensity profiles for the diode with thick or thin ␤-Ga2 O3 layer. Because the absorption coefficient of ␤-Ga2 O3 is about 105 cm−1 , intensity of UV light is decreased to e−1 at a depth of 100 nm. Therefore, small amount of UV light reaches to the depletion region of ␤-Ga2 O3 /GaN heterojunction with thick ␤-Ga2 O3 layer comparing with the diode with thin ␤Ga2 O3 layer. Because the carrier generated in the depletion layer of the diode with thick ␤-Ga2 O3 layer is small, photo-current should be small. We think this is a reason why the diode with 116 nm ␤Ga2 O3 layer had higher responsivity than the diode with 175 nm ␤-Ga2 O3 layer.

(4)

where De and Dh is diffusion coefficient of electron and hole, Le and Lh is diffusion length of electron and hole. The n30 and p20 is minority carrier density of GaN and ␤-Ga2 O3 , respectively. These currents in three regions are the same. Electrons and holes generated by the absorption of deep UVlight both in the depletion layer of ␤-Ga2 O3 under bias and in the resistance region of ␤-Ga2 O3 contribute to the photocurrent of the diode. This gives rise to a higher responsivity at wavelengths shorter than 240 nm. When the deep UV light is illuminated on the diode, n20 and p20 is increased to n20 +  20 and p20 + p20 in the resistance region, also n30 and p30 are increased to n30 + n30 and p30 + p30 in the GaN layer. In the depletion layer in ␤-Ga2 O3 side of pn junction, electron and hole pair of n20 and p20 is generated. Generation in the depletion layer in GaN side of pn junction can be ignored because of the thinner depletion layer in GaN side. Although the generation depends on the intensity of the light at the position, it was assumed that the intensity is uniform. The photogenerated holes in the ␤-Ga2 O3 layer flow to the GaN layer through the pn junction; however, the photogenerated electrons flow to the Au electrode over the forward-biased Schottky barrier. Because the minority carrier density p20 in ␤-Ga2 O3 and n30 in GaN is also increased to p20 + p20 and n30 + n30 , the reverse current of pn junction (Eq. (4)) is increased to I + I. An increase of n20 and p20 in resistance region also leads to an increase of current (Eq. (3)). If an influence of photogenerated hole at the interface of Schottky junction is ignored, the equation of current of Eq. (2) is independent of light. Therefore, an increase of the current means

4. Conclusions In conclusion, a ␤-Ga2 O3 layer was prepared on a p-type GaN template substrate by gallium evaporation in oxygen plasma, which formed a heterojunction photodiode using a planar technique by a three-step mask process. The ␤-Ga2 O3 layer had (−2 0 1) oriented crystal structure on (0 0 1) GaN. The photodiode can detect deep-UV light at wavelengths less than 240 nm. The response characteristics demonstrated were also better than those for photodiodes based on the ␤-Ga2 O3 /SiC heterojunction reported in our previous study. These results indicate that ␤-Ga2 O3 /GaN heterojunction photodiodes are promising for the detection of UV light. Acknowledgements This work was partly supported by a Grant from the Research Center for Creative Partnerships of Ishinomaki Senshu University. References [1] Y. Kokubun, K. Miura, F. Endo, S. Nakagomi, Sol–gel prepared ␤-Ga2 O3 thin films for ultraviolet photodetectors, Appl. Phys. Lett. 90 (2007) 31912. [2] T. Oshima, T. Okuno, N. Arai, N. Suzuki, S. Ohira, S. Fujita, Vertical Solar-blind deep-ultraviolet schottky photodetectors based on ␤-Ga2 O3 substrates, Appl. Phys. Express 1 (2008) 11202. [3] T. Oshima, T. Okuno, N. Arai1, N. Suzuki, H. Hino, S. Fujita, Flame detection by a ␤-Ga2 O3 -based sensor, Jpn. J. Appl. Phys. 48 (2009) 011605. [4] R. Suzuki, S. Nakagomi, Y. Kokubun, Solar-blind photodiodes composed of a Au Schottky contact and ␤-Ga2 O3 single crystal with a high resistivity cap layer, Appl. Phys. Lett. 98 (2011) 131114. [5] S. Nakagomi, T. Momo, S. Takahashi, Y. Kokubun, Deep ultraviolet photodiodes based on ␤-Ga2 O3 /SiC heterojunction, Appl. Phys. Lett. 103 (2013) 72105. [6] H.J. Lee, T.I. Shin, D.H. Yoon, Influence of NH3 gas for GaN epilayer on ␤-Ga2 O3 substrate by nitridation, Surf. Coat Technol. 202 (2008) 5497–5500.

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