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Characterizations of bismuth telluride/gallium nitride heterojunction photovoltaic detector for MWIR detection under room temperature

Uncooled infrared detectors have aroused keen interest in narrow bandgap semiconductor research. We propose to use Bismuth Telluride (Bi2Te3) bandgap energy of about 0.15 eV for middle wavelength IR (MWIR) detection. In this work, a heterojunction structure of Bi2Te3/GaN/Al2O3 was fabricated to form a photovoltaic detector. The structural properties of Bi2Te3 thin films were characterized by high resolution x-ray diffraction spectroscopy. Crystallinity of Bi2Te3 thin films was found to be strongly dependent on deposition temperatures. The spectral current responsivity for this heterojunction in NIR-to-MWIR region was evaluated under back-side illumination. The devices were also characterized by studying their low-frequency noise spectra measurement. The experimental results suggest that, the noise level of the devices is correlated to the crystallinity of the Bi2Te3 thin films.


Introduction
Many physical phenomena that respond to infrared (IR) photon energy in the range of about 0.1 to 1 eV have been exploited for making IR photon detectors [1]. Mercury cadmium telluride (HgCdTe) has remained the most important semiconductor for middle to long infrared detection in the past three decades. Numerous studies attempt to replace HgCdTe by alternative materials such as mercury-based alloys (HgZnTe, HgMnTe), PbSnTe and AlGaAs [1][2][3][4]. In this respect we propose to use thin film of narrow band gap bismuth telluride for infrared detection. Bismuth telluride (Bi 2 Te 3 ), a V-VI semiconducting alloy, is well known in thermoelectricity field for its outstandingly high Seebeck coefficient at room temperature [5]. Bi 2 Te 3 and its derivatives Bi 2-x Sb x Te 3 and Bi 2 Te 3-y Se y are the most commonly adopted semiconductors in recent thermoelectric devices. The Bi 2 Te 3 is a narrow band gap semiconductor with layered structure. The crystal structure of Bi 2 Te 3 -based compounds composes of atomic layers in the order of Te/Bi/Te/Te/Bi/Te/Bi/Te/Te/… oriented along the c-axis. The Te/Te layers are held together by weak van der Waals forces [6]. Theoretical and experimental values of band gap energy for Bi 2 Te 3 are about 0.11 eV and 0.15 eV respectively [7].
In this work, a novel heterojunction photovoltaic detector structure of Bi 2 Te 3 /GaN/Al 2 O 3 was fabricated for MWIR detection. High quality n-GaN epitaxial layers were first grown on sapphire substrates by MOCVD technique. Then Bi 2 Te 3 thin films were subsequently grown on the n-GaN/Al 2 O 3 template by pulsed laser deposition (PLD) technique. PLD is one of the most versatile physical vapour deposition techniques which is able to transfer the composition of the target materials directly on to the thin films [6,8]. The laser plume produced under ablation is consists of materials very close to the stoichiometry of the target. It is feasible to use PLD to control the films quality with a variety of structures ranging from polycrystalline to textured growth and multiple stoichiometry of Bi x Te 1-x thin films. At present, only few studies on PLD growth Bi 2 Te 3 thin films were reported in the literature [8][9][10]. Our works demonstrates the influences of the deposition temperature on the crystallinity of Bi 2 Te 3 thin films. GaN, due to its wide band gap energy of 3.4 eV at room temperature, is a good MWIR blind material. It acts as both the n-type semiconducting layer in the p-n junction and as a window for IR transmission in a back-side illuminated diode configuration. Besides, replacing the n-type layer by the wide band gap semiconductor will contribute to some advantages over conventional narrow band gap semiconductor homojunction. GaN has much higher thermal stability than most of the narrow band gap materials; thus, it will exhibit much lower equilibrium concentration of thermionic minority carriers under room temperature operation. As a result, lower leakage current in reverse bias branch. In addition, IR radiation is transparent to the wide band gap semiconductor; therefore, the efficiency for the photons to reach the active region (depletion layer) will also be enhanced [11].
Since the performance of a photovoltaic detector is greatly governed by its I-V characteristics, we have measured room temperature the p-Bi 2 Te 3 /n-GaN heterojunction I-V characteristics. The variety of p-Bi 2 Te 3 layers were grown under control of the deposition temperature, their corresponding diode parameters were determined by employing real diode equation. Low-frequency noise is shown to arise from the trapping and detrapping of charge carries through thermally activated process, therefore, normalized current noise power spectra were studied in order to clarify the materials quality for the devices. The characterizations of electrical and optical measurements are correlated to the structural properties of the PLD growth Bi 2 Te 3 semiconductor.

Experimental details
Bi 2 Te 3 thin films have been deposited by pulsed laser deposition (PLD) on MOCVD grown (00n) n-GaN/sapphire. The thickness of the n-GaN layer is about 2μm. A commercial Bi 2 Te 3 (99.999% ACI ALLOYS) PLD target used. Before the deposition process, the experimental chamber was evacuated to a base pressure of ~3 x 10 -5 Torr by diffusion pump. The n-GaN/sapphire substrate was slowly heated up by an ohmic heater embedded in the substrate holder. A KrF excimer laser (COMPex 205, Lambda Physik) operating at 10 Hz repetition rate was employed. The on-target pulsed laser energy density was about 5 Jcm -2 and each film deposition lasted for 10 min. The as-prepared films were further in-situ annealed under the same deposition ambient and temperature for 30 min. The thickness of the films was measured by imaging the cross section of the p-Bi 2 Te 3 /n-GaN interface with a scanning electron microscope (SEM). Typically a ~500 nm thick film is generated for a deposition time of 10 min.
The PLD growth photodiode was fabricated by using shadow mask technique. Two separate stainless steel masks containing circular holes of diameter of 2 mm and 4 mm were used to deposit ohmic contact Au on Bi 2 Te 3 and Bi 2 Te 3 thin films on n-GaN/A1 2 O 3 respectively. The ohmic contact for n-GaN is obtained by depositing Ti metal with a 9 mm diameter annular aperture. The crystal structure of the heterojunction was examined by x-ray diffractometer (XRD) (Bruker D8 Discover, CuK α radiation) configured in a θ-2θ scan mode. A JSM-6335F Field Emission Scanning Electron Microscope (SEM) operating at accelerating voltage of 3 kV was applied to study the morphology of the films. The I-V characteristics of the heterojunction were investigated via electrical measurements using a HP 4140B picoammeter.
Spectral responsivity of Bi 2 Te 3 heterojunction detector is obtained using a 300 W halogen lamp and placed in front of an Oriel 74004 monochromator, which contains two diffraction gratings over the range from near to long infrared region. The output photocurrent was subsequently measured by an HP 617 picoammeter.

Experimental results and discussions
X-ray diffraction characterizations of Bi 2 Te 3 thin film on GaN/sapphire substrate deposited at temperature range from 150 to 300 ℃ were conducted, in order to determine their structural parameters. The temperature dependent of X-ray diffraction pattern of the Bi rich-Bi 2 Te 3 /GaN/sapphire is depicted in Figure 1(a)-(d) in logarithmic scale. It is observed that all Bi 2 Te 3 thin films are polycrystalline in nature [6,10]. Crystal structure of various Bi 2 Te 3 thin films are belongs to trigonal system with an equivalent hexagonal unit cell. By investigating the crystallinity (orientation and grain size) among the sample with different deposition temperature, the influence of the increase in deposition temperature to the crystallinity of the Bi 2 Te 3 thin films can be pointed out. In general, there is one strong diffraction peak within each diffraction pattern in Fig. 1(a)-(d), along the plane (015). Besides, the enhancement of crystallinity of the thin films in terms of averaged grain size is also obtained from the XRD spectrum. There is only one well-defined (015) peak in Fig. 1(a). At 250℃, the crystallinity of the films is improved and the (02 10) peak is shown. However, only sample grown at 300℃ has shown peak corresponding to c-oriented (00 15) plane of Bi 2 Te 3 in hexagonal phase. From Fig. 1(d), the lattice parameters of the films were calculated using Bragg's formula. The lattice parameters deduced from the diffraction angle are a H = 0.443nm and c H = 2.98nm. Comparing these values to the values available in the literature [12] and ASTM data (a H = 0.438nm and c H = 3.048nm) [Card No. 15-0863], there is a noticeable variation in the lattice constants. The small reduction of the lattice constant c H for the deposited thin film maybe indicates that the film is in contraction or a small amount of Te occupies Bi sites [8,13]. Due to the large difference in vapour pressure of telluride relative to bismuth, the thin films that we obtained may demonstrate of the segregation of bismuth. A small deviation of stoichiometric composition of Bi 2 Te 3 to Bi rich samples will result in an effective p-type doping effect on the material.
By measuring the full-width-half maximum of (015) XRD reflections in Fig. 1(a)-(d) and adopting the Scherrer formula [12], an averaged grain size of around 100 to 200 nm can be estimated for the Bi 2 Te 3 films grown under 150-300℃ deposition temperatures.  The temperature dependence of the surface morphology is presented in Fig. 2(a)-(d). The films deposited on GaN/Al 2 O 3 at 150℃ (Fig. 2(a)) is relatively smooth with nano size crystals. When increasing the deposition temperature to 200℃, 250℃ and 300℃, the surface morphology of the films are found to be rougher than those grown at deposition temperatures and composed of high density crystals. At 200℃ the film surface is consists of 100-300 nm size grains; at 250℃ the grains size is increased to 500-1000 nm; the crystals appear on the film grown at 300℃ is quite large with few micrometer size. Moreover, it clearly exhibits the layered structure of Bi 2 Te 3 with randomly oriented crystals. This is also conformed by the XRD spectra ( Fig. 1(d)) where both [m 0 n] and [0 0 n] direction peaks are found, confirming the less preferential growth take place at such high substrate temperature. The I-V characteristics of all devices were carried out in order to determine their junction parameters, the experimental results are shown in Fig. 3. Bi 2 Te 3 diode were fabricated on n-GaN/Al 2 O 3 to form a p-n heterojunction, which illustrates reasonably good p-n junction characteristics with low reverse bias current density and small series resistance in forward bias region. In Fig. 3, it is obvious that the reverse bias current increases slowly with the deposition temperature. The performance of various devices can be justify using the incremental differential resistance R 0 = ( dV/dI ) V=0 [14]. The values of R 0 were obtained by differentiating the I-V curve at zero bias voltage. All values are summarized in table 1 for comparison. This quantitative calculation of R 0 illustrates a trend in the variation of electrical characteristics between the four devices. The  It has been shown that the low-frequency noise in semiconductor device is an effective tool to investigate the trapping and detrapping states of interface trap density. The low-frequency excess noise spectrum for p-Bi 2 Te 3 /n-GaN/Al 2 O 3 photodiode fabricated under different substrate temperature is measured. The current noise power spectral density, S I (f), is given by: where I is the dc current at constant bias voltage, R is the ac resistance of the device, R Δ is the inside the junction, and it is directly proportional to the trap density as well. Nearly three orders of reduction in the normalized current noise power density (Fig. 4) are obtained for p-Bi 2 Te 3 /n-GaN photodiode fabricated under 300℃ and 350℃. Our results indicate that the junction electronic quality is improved by choosing optimal substrate temperature, which contribute to the better crystallinity in terms of their average grain size.  The preliminary spectral response for Bi 2 Te 3 photovoltaic detectors is shown in Fig. 5. Overall responsivity of these devices is improved with increasing deposition temperature. The devices are relatively more sensitive at wavelength about 6 µm. Their responsivity at this particular wavelength is summarized in table 1 for better comparison. It is important to study the correlation between the effects of the structural and electrical properties on the optical response. We found that the effect of the improved crystallinity of the p-Bi 2 Te 3 layer may result in the improvement of responsivity in middle IR region. In contrast, higher deposition temperature may result in sacrificing the electrical properties which introduces larger reverse bias leakage current.

Conclusions
We performed a systematic study on the structural, electrical and optical properties of PLD-grown p-Bi 2 Te 3 /n-GaN/Al 2 O 3 heterojunctions. The crystal quality is enhanced by increasing the deposition temperature from 150 to 300℃. By varying the deposition temperature, crystal domain size is able to have a better control. On the other hand, excessively high deposition temperature may result in the degradation of the electrical properties. The I-V curves reveal a larger reverse bias leakage current. In low-frequency noise measurement, the reduction in current noise power density for the device fabricated at 300 ℃ exhibit the most sensitive responsivity in middle IR region. Our results demonstrate the p-Bi 2 Te 3 /n-GaN/Al 2 O 3 heterojunction photodetector can be utilized in middle IR detection.