Deep Ultraviolet Photodetectors Research Articles

Growth of β-Ga2O3 Single-Crystal Microbelts by the Optical Vapor Supersaturated Precipitation Method

Monoclinic β-Ga2O3 microbelts were successfully fabricated using a one-step optical vapor supersaturated precipitation method, which exhibited advantages including a free-standing substrate, prefect surface, and low cost. The as-grown microbelts possessed a well-defined geometry and perfect crystallinity. The dimensions of individual β-Ga2O3 microbelts were a width of ~50 μm, length of ~5 mm, and thickness of ~3 μm. The SEM, XRD, HRTEM, XPS, and Raman spectra demonstrated the high single-crystalline structure of β-Ga2O3 microbelts. Twelve frequency modes were activated in Raman spectra. The optical band gap of the β-Ga2O3 microbelt was calculated to be ~4.45 eV. Upon 266 nm excitation, 2 strong UV emissions occurred in photoluminescence spectra through the radiative recombination of self-trapped excitons, and the blue emission band was attributed to the presence of donor-acceptor-pair transition. The individual β-Ga2O3 microbelt was employed as metal-semiconductor-metal deep-ultraviolet photodetector, which exhibits the photoresponse under 254 nm. This work provides a simple and economical route to fabricate high-quality β-Ga2O3 single-crystal microbelts, which should be a potential synthetic strategy for ultra-wide bandgap semiconductor materials.

High performance solar-blind deep ultraviolet photodetectors via β-phase (In0.09Ga0.91)2O3 single crystalline film

We successfully fabricate a high performance β-phase (In0.09Ga0.91)2O3 single-crystalline film deep ultraviolet (DUV) solar-blind photodetector. The 2-inches high crystalline quality film is hetero-grown on the sapphire substrates using the plasma-assisted molecular beam epitaxy (PA-MBE). The smooth InGaO single crystalline film is used to construct the solar-blind DUV detector, which utilized an interdigitated Ti/Au electrode with a metal–semiconductor–metal structure. The device exhibits a low dark current of 40 pA (0 V), while its UV photon responsivity exceeds 450 A/W (50 V) at the peak wavelength of 232 nm with illumination intensity of 0.21 mW/cm2 and the UV/VIS rejection ratio (R 232 nm/R 380 nm) exceeds 4 × 104. Furthermore, the devices demonstrate ultrafast transient characteristics for DUV signals, with fast-rising and fast-falling times of 80 ns and 420 ns, respectively. This excellent temporal dynamic behavior can be attributed to indium doping can adjust the electronic structure of Ga2O3 alloys to enhance the performance of InGaO solar-blind detectors. Additionally, a two-dimensional DUV scanning image is captured using the InGaO photodetector as a sensor in an imaging system. Our results pave the way for future applications of two-dimensional array DUV photodetectors based on the large-scale InGaO heteroepitaxially grown alloy wide bandgap semiconductor films.

Fully-transparent self-powered ultraviolet photodetector based on GaOx/ZnO heterojunction for solar-blind imaging

With the development of fully-transparent electronic materials and energy-efficient photodetectors, self-powered deep-ultraviolet photodetectors that are fully transparent are expected to have a wide range of applications in solar-blind imaging. Here, we present the first investigation of a self-powered, fully transparent Ga2O3 deep-ultraviolet photodetector array for solar-blind imaging. The device is composed of an amorphous Ga2O3/ZnO heterojunction (GaOx/ZnO) with indium tin oxide (ITO) transparent electrodes, which was prepared using a fully magnetron sputtering method. The fully-transparent device demonstrates exceptional performance at both − 5 V bias and 0 V bias, with a high responsivity of 6.5 × 103 A/W and 13 mA/W, respectively, coupled with excellent stability. Moreover, the device is self-powered due to the built-in electric field formed at the interface of GaOx and ZnO, which efficiently drives carrier motion and generates photocurrent. Finally, we demonstrate the feasibility of transparent devices for solar-blind imaging at 0 V bias by constructing a 5 × 4 GaOx/ZnO heterojunction solar-blind photodetector array, which produces a high-contrast image of the target object. This work provides a new strategy for developing Ga2O3-based fully-transparent self-powered photodetector arrays for solar-blind imaging.

Non-ultrawide bandgap CsPbBr3 nanosheet for sensitive deep ultraviolet photodetection

In this study, we report the fabrication of a sensitive deep ultraviolet (DUV) photodetector by using CsPbBr3 nanosheets synthesized through confined-space growth at room temperature. The peak photoresponse of the device upon illumination blueshifts with decreasing thickness and a 68 nm CsPbBr3 nanosheet based device displays a peak response to 265 nm illumination, showing responsivity and specific detectivity of 85 mA/W and 4.05 × 1011 Jones at 3 V bias, respectively. Theoretical simulation reveals that the blueshift is associated with the wavelength-dependent absorption coefficient of CsPbBr3 nanosheet: incident light with shorter wavelengths can be absorbed on the superficial surface, while long-wavelength light has a larger penetration depth, leading to dominant DUV absorption in thinner CsPbBr3 nanosheet. This work will shed light on the facile and cost-effective fabrication of DUV photodetectors from non-ultrawide bandgap (UWBG) all-inorganic perovskite materials.

Rapid Response Solar Blind Deep UV Photodetector with High Detectivity Based On Graphene:N/βGa2 O3 :N/GaN p-i-n Heterojunction Fabricated by a Reversed Substitution Growth Method.

This work reports a high-detectivity solar-blind deep ultraviolet photodetector with a fast response speed, based on a nitrogen-doped graphene/βGa2 O3 /GaN p-i-n heterojunction. The i layer of βGa2 O3 with a Fermi level lower than the central level of the forbidden band of 0.2eV is obtained by reversed substitution growth with oxygen replacing nitrogen in the GaN matrix, indicating the majority carrier is hole. X-ray diffractometershows that the transformation of GaN into βGa2 O3 with (-201) preferred orientation at temperature above 900°C in an oxygen ambient. The heterojunction shows enhanced self-powered solar blind detection ability with a response time of 3.2µs (rise)/0.02ms (delay) and a detectivity exceeding 1012 Jones. Under a reverse bias of -5V, the photoresponsivity is 8.3 A W-1 with a high Ilight /Idark ratio of over 106 and a detectivity of ≈9 × 1014 Jones. The excellent performance of the device is attributed to 1) the continuous conduction band without a potential energy barrier, 2) the larger built-in potential in the heterojunction because of the downward shift of Fermi energy level in β-Ga2 O3 , and 3) an enhanced built-in electric field in the βGa2 O3 due to introducing p-type graphene with a high hole concentration of up to ≈1020 cm-3 .

In-situ composition development of Zn/In-doped Ga2O3 nanowire with ultrahigh responsivity and long-term stability for deep-UV photodetector

High-performance deep ultraviolet photodetectors (UV PDs) based on Ga2O3 have been applied widely in civil and military exploration. The incident photon-to-electron conversion efficiency of Ga2O3-based UV PDs could be improved by doping metal or non-metal elements in the lattice. Herein, multi-metallic oxide of InZn co-doped Ga2O3 (InZn-Ga2O3) nanowires is synthesized via the in-situ chemical vapor deposition (CVD) method. The favorable 25%InZn-Ga2O3 exhibits a maximum responsivity of 440.42 A W−1 and a peak external quantum efficiency (EQE) of 2.15 × 105% under 254 nm light illumination at 10 V, which are 1.19 × 104 times higher than that for pure Ga2O3, indicating its enhanced detection ability for deep-UV light and high conversion efficiency from photons to electrons. A high UV/visible rejection ratio of 7.3 × 105 is also obtained, implying a high sensitivity for UV light. Moreover, the responsivity of the InZn-Ga2O3 UV PD remains unchanged after a long period of 8000 s testing. The superior performances of InZn-Ga2O3 UV PD are attributed to the improved crystal quality with decreased oxygen vacancies and elevated conductivity, which facilitate the separation and transfer of photoinduced electron-hole carriers. The DFT calculation and UV–vis spectra results reveal that the InZn-Ga2O3 possesses a narrowed band gap with enhanced light absorption and a decreased work function for the fast escape of electrons. In addition, the ohmic contact between InZn-Ga2O3 and Ag electrode accelerates the transfer and diffusion of photogenerated electrons. The achievements in our work present a multi-metallic oxide nanowire with high performance and an innovative mechanism analysis method for the applications of optoelectronic devices.

High-temporal dynamic β-Ga203 ultraviolet solar-blind photodetector via heteroepitaxy single crystalline film

In this report, we successfully fabricate a high performance β-phase Ga203 single-crystalline film deep ultraviolet (DUV) solar-blind photodetector. The hybrid Ga/GaO buffer layer method is proposed to promote the quality of β-Ga203 film that hetero-grown on sapphire substrates using plasma-assisted molecular beam epitaxy (PA-MBE). Based on the 2-inch and smooth β-Ga203 single crystalline film, the solar-blind detector is constructed by interdigitated Au-electrode with metal-semiconductor-metal structure. The dark-state current of device is low as 40 pA, meanwhile the UV photon responsibility > 234 mA/W at peak of 219 nm. Moreover, the device exhibites an ultrafast transient characteristic for DUV signals with a fast-rising time of 3 μs and decay time of 23 μs. High contrast two-dimension scanning image has been recorded using such DUV photodetector as a sensor in an imaging system. The excellent performances of device could be attributed to the high crystalline quality of β-Ga2O3, and giant field mobility in Schottky junction. Our results present a significant step towards future application of two-dimension array DUV photodetector based on larger scale β-Ga203 heteroepitaxy film.

Metal–Semiconductor–Metal Photodetectors Based on β-MgGaO Thin Films

Ultrawide-bandgap (UWBG) deep-ultraviolet photodetectors have received great attention due to their versatile applications in the fields of scientific research, civilian infrastructure, military defense, etc. In this perspective, we fabricated deep-ultraviolet β-MgGaO metal–semiconductor–metal photodetectors with interdigital Pt/Au metal contacts. β-Phase MgGaO ternary alloy thin films of different Mg atomic percentages were grown using oxygen plasma-assisted molecular beam epitaxy. Ultrawide bandgaps of 5.03, 5.09, 5.15, and 5.22 eV were achieved for thin films with and without Mg2+ incorporation, and light transmittances of all samples were around 90% in the visible region. Raman spectra indicate that Mg2+ atoms have replaced the position of Ga3+ ions in both octahedral and tetrahedral chains. The responsivity of the detectors was investigated. The irradiation wavelength-, temperature-, and power-dependent I–V curves, photocurrent spectra, and dynamics of the photocurrents were measured. This work suggests that UWBG β-MgGaO semiconductors have a potential for deep-ultraviolet photodetectors and other photonic device applications.

Wafer-Scale Single Crystal Hexagonal Boron Nitride Layers Grown by Submicron-Spacing Vapor Deposition.

The direct growth of wafer-scale single crystal two-dimensional (2D) hexagonal boron nitride (h-BN) layer with a controllable thickness is highly desirable for 2D-material-based device applications. Here, for the first time, a facile submicron-spacing vapor deposition (SSVD) method is reported to achieve 2-inch single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers on the dielectric sapphire substrates using a boron film as the solid source. In the SSVD growth, the boron film is fully covered by the same-sized sapphire substrate with a submicron spacing, leading to an efficient vapor diffusion transport. The epitaxial h-BN layer exhibits extremely high crystalline quality, as demonstrated by both a sharp Raman E2g vibration mode (12 cm-1 ) and a narrow X-ray rocking curve (0.10°). Furthermore, a deep ultraviolet photodetector and a ZrS2 /h-BN heterostructure fabricated from the h-BN layer demonstrate its fascinating properties and potential applications. This facile method to synthesize wafer-scale single crystal h-BN layers with controllable thickness paves the way to future 2D semiconductor-based electronics and optoelectronics.

Oxygen-vacancy-dependent high-performance α-Ga2O3 nanorods photoelectrochemical deep UV photodetectors

Ga2O3 is a good candidate for deep ultraviolet photodetectors due to its wide-bandgap, good chemical, and thermal stability. Ga2O3-based photoelectrochemical (PEC) photodetectors attract increasing attention due to the simple fabrication and self-powered capability, but the corresponding photoresponse is still inferior. In this paper, the oxygen vacancy (Vo) engineering towards α-Ga2O3 was proposed to obtain high-performance PEC photodetectors. The α-Ga2O3 nanorods were synthesized by a simple hydrothermal method with an annealing process. The final samples were named as Ga2O3-400, Ga2O3-500, and Ga2O3-600 for annealing at 400 ℃, 500 ℃, and 600 ℃, respectively. Different annealing temperatures lead to different Vo concentrations in the α-Ga2O3 nanorods. The responsivity is 101.5 mA W−1 for Ga2O3-400 nanorod film-based PEC photodetectors under 254 nm illumination, which is 1.4 and 4.0 times higher than those of Ga2O3-500 and Ga2O3-600 nanorod film-based PEC photodetectors, respectively. The photoresponse of α-Ga2O3 nanorod film-based PEC photodetectors strongly depends on the Vo concentration and high Vo concentration accelerates the interfacial carrier transfer of Ga2O3-400, enhancing the photoresponse of Ga2O3-400 nanorod film-based PEC photodetectors. Furthermore, the α-Ga2O3 nanorod film-based PEC photodetectors have good multicycle, long-term stability, and repeatability. Our result shows that α-Ga2O3 nanorods have promising applications in deep UV photodetectors.

Low-temperature spatial atomic layer deposited Ga2O3 films for high-performance flexible deep ultraviolet photodetector

This research aims to prepare amorphous gallium oxide (α-Ga2O3) films for use in flexible deep ultraviolet photodetectors. The films are deposited using high-speed spatial atomic layer deposition (SALD) at different substrate temperatures. The experimental results show that the saturating deposition rate at 145–155 °C is four to sixteen times higher than that of temporal ALD. Moreover, under the saturated SALD conditions the deposited film has less carbon impurities, oxygen-related defects, and greater compactness than under non-saturated conditions. Finally, a high-performance flexible deep ultraviolet α-Ga2O3 photodetector with fast decay time of 161 µs and low dark current density of 6.8 × 10-10 A/cm2 is achieved.

Nanoscale-Thick CuPc/β-Ga2O3 p–n Junctions for Harsh-Environment-Resistant Self-Powered Deep-UV Photodetectors

Due to their crucial role in ultraviolet communication and monitoring, deep-ultraviolet (DUV) photodetectors have garnered much interest. Recently, Ga2O3 has emerged as the best material for DUV photodetectors because of its ultrawide bandgap (4.5–4.9 eV), excellent UV photon absorption coefficient, high structural stability, and affordability. However, there are several difficulties in realizing high-performance Ga2O3-based DUV photodetectors with a high tolerance for harsh environments. In this work, nanoscale-thick CuPc/β-Ga2O3 p–n junctions were used to build high-performance DUV photodetectors by a straightforward solution-processing approach. The p–n junction photodetectors exhibit improved photoelectric performance compared to a single device made of β-Ga2O3 or CuPc, with a photo-to-dark current ratio of 3700 and a fast response time of ∼20 ms under a bias of 0 V. Due to the excellent stability of the nanoscale-thick CuPc film, the device can maintain a high photocurrent even at high temperatures or under long-term DUV irradiation. Our work provides an effective strategy toward highly harsh-environment-resistant DUV photodetectors.

Directional Carrier Transport in Micrometer-Thick Gallium Oxide Films for High-Performance Deep-Ultraviolet Photodetection.

Incorporating emerging ultrawide bandgap semiconductors with a metal-semiconductor-metal (MSM) architecture is highly desired for deep-ultraviolet (DUV) photodetection. However, synthesis-induced defects in semiconductors complicate the rational design of MSM DUV photodetectors due to their dual role as carrier donors and trap centers, leading to a commonly observed trade-off between responsivity and response time. Here, we demonstrate a simultaneous improvement of these two parameters in ε-Ga2O3 MSM photodetectors by establishing a low-defect diffusion barrier for directional carrier transport. Specifically, using a micrometer thickness far exceeding its effective light absorption depth, the ε-Ga2O3 MSM photodetector achieves over 18-fold enhancement of responsivity and simultaneous reduction of the response time, which exhibits a state-of-the-art photo-to-dark current ratio near 108, a superior responsivity of >1300 A/W, an ultrahigh detectivity of >1016 Jones, and a decay time of 123 ms. Combined depth-profile spectroscopic and microscopic analysis reveals the existence of a broad defective region near the lattice-mismatched interface followed by a more defect-free dark region, while the latter one serves as a diffusion barrier to assist frontward carrier transport for substantially enhancing the photodetector performance. This work reveals the critical role of the semiconductor defect profile in tuning carrier transport for fabricating high-performance MSM DUV photodetectors.

Improved Performance of Perovskite Deep-Ultraviolet Photodetector Using FAPb(I/Br)3 as Light Absorption Layer

Constitutive engineering by adding halide anions is one effective way to improve the performance of photodetectors by adjusting the bandgap. In this study, a mixed-anion perovskite thin film was facile fabricated by post-processing of a pure FAPbI3 film with a formamidinium bromide (FABr) solution. In addition, the manufactured thin film was used as the light absorption layer, SnO2-SDBS as the electron transport layer, and spiro-OMeTAD as the hole injection layer to fabricate a deep ultraviolet(UV) photodetector. The device exhibited a response of 43.8 mA/W−1, a detectability of 3.56 × 1013 Jones, and an external quantum efficiency of 38%. Therefore, this study is promising for various applications in the deep-UV wavelength region.

Pulsed laser deposition of ZnGa2O4 thin films on Al2O3 and Si substrates for deep optoelectronic devices applications

Due to the ultra-wideband gap, high transmittance in the ultraviolet (UV) region, and excellent environmental stability, zinc gallium oxide (ZnGa2O4) has attracted considerable interest in deep-ultraviolet photodetectors. Here, ZnGa2O4 thin film was fabricated on different substrates by pulsed laser deposition with a post-annealing process under an oxygen atmosphere. It is found that the substrates have a great impact on the morphology, structure, and crystal quality of thin film. After annealing, the thin film quality has been improved. The metal–semiconductor–metal photodetector shows excellent reproducible characteristics and fast response performance, which demonstrates great potential in next-generation optoelectronic devices.

Influence of oxygen vacancy and metal–semiconductor contact on the device performance of amorphous gallium oxide photodetectors

Here we investigate the influence of the oxygen vacancy content and the electrode contact on the performance of deep ultraviolet photodetectors based on amorphous Ga2O3 films. The fine-tuning of the oxygen ratio effectively reduces the oxygen vacancy content, which obtains optimized device performance with a responsivity of 5.78 A W−1 and a rise/fall time of 301/89 ms. The metal contact formation and its impact on the device performance are further studied. Compared to the Ohmic-type device using Ti electrodes, the Schottky-type devices using Au and Al electrodes exhibit a shorter rise time and a lower dark current.

One ε-Ga2O3-based solar-blind Schottky photodetector emphasizing high photocurrent gain and photocurrent-intensity linearity

The ε-Ga2O3 thin film was grown on sapphire substrate by using metalorganic chemical vapor deposition (MOCVD) method, and then was used to fabricate a deep-ultraviolet (DUV) photodetector (PD). The ε-Ga2O3 thin film shown good crystal quality and decent surface morphology. Irradiated by a 254-nm DUV light, the photodetector displayed good optoelectronic performance and high wavelength selectivity, such as photoresponsivity (R) of 175.69 A/W, detectivity (D*) of 2.46 × 1015 Jones, external quantum efficiency (EQE) of 8.6 × 104% and good photocurrent-intensity linearity, suggesting decent DUV photosensing performance. At 5 V and under illumination with light intensity of 800 μW/cm2, the photocurrent gain is as high as 859 owing to the recycling gain mechanism and delayed carrier recombination; and the photocurrent gain decreases as the incident light intensity increases because of the recombination of photogenerated carriers by the large photon flux.

Deep Ultraviolet Photodetector with Ultrahigh Responsivity based on a Nitrogen‐Doped Graphene‐Modified Polypyrrole/SnO2 Organic/Inorganic p–n Heterojunction

AbstractDeep ultraviolet monitoring is realized via a high crystal quality SnO2 microwire (MW)‐based photodetector (PD). This is then combined with 2D nitrogen‐doped graphene (NGr), conducting polymer polypyrrole (PPy), and an in situ polymerization‐fabricated composite film PPy‐NGr to construct an organic–inorganic p–n heterojunction PD. The long response time brought on by the oxygen adsorption of SnO2 MW is greatly decreased via coating with the aforementioned materials. A defect response is created by the surface dangling bonds of SnO2 MW, which can be effectively suppressed by the PPy. Absorption in the deep ultraviolet region (<240 nm) by PPy results in a blue shift of the response peak of the PPy/SnO2 heterojunction PD compared to that of a single SnO2 PD. The introduction of NGr improves the detection performance by providing a smoother energy band migration to reduce photogenerated carrier recombination and stacking at the potential barrier. The ultrahigh responsivity of PPy‐NGr/SnO2 PD is 4594.25 A W−1 and the detectivity is 6.47 × 1011 Jones, 40 and nine times greater, respectively, than those of a PPy/SnO2 PD under a 5 V reverse bias and 240‐nm light irradiation (18.75 µW cm−2). The novel strategy provides a reference for the future design of high‐performance heterojunction PDs.

Study on a eco-friendly and efficient method for growing aluminum nitride whiskers

As the III-V semiconductor with the highest band gap, aluminum nitride (AlN) has broad application prospects in the deep ultraviolet (UVC) field. AlN have unparalleled advantages in the fabrication of nanodevices, making the future of aluminum nitride whiskers bright. The combination of deep-UV selectivity and low-voltage operation makes AlN nanowires the best candidate for developing compact deep-UV photodetectors. In this article, a high-efficiency, eco-friendly AlN whisker preparation technique is introduced on (0001) sapphire substrate without metal catalysts. The as-obtained AlN whiskers have hexagonal wurtzite structure and zigzag morphology, the diameter can reach below 1–10 μm（μm）, and the lengths are tens of microns. The AlN whiskers grows along the [100] direction, but the crystal orientation of growth can be changed by adjusting the V/III ratio. The possible growth mechanism of AlN whiskers is described in the report.

Dual-band and dual-mode ultraviolet photodetection characterizations of Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N homo-type heterojunction

The deep-ultraviolet (DUV) photodetectors (PDs) have important applications in lots of fields. Thus, developing self-powered DUV PDs and excavating the inherent mechanism seem seriously crucial to achieving further actual applications. The construction of heterojunction can lead to many desired characteristics in optoelectronic devices. In the field of DUV photodetection, Ga<sub>2</sub>O<sub>3</sub> has been a popular subject for constructing DUV PDs. So, it is necessary to develop self-powered Ga<sub>2</sub>O<sub>3</sub>-based DUV PDs through fabricating its heterogeneous structure. Therefore, in this work, the Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction DUV PD is fabricated and discussed, which can achieve 254 and 365 nm DUV light photodetection. At positive voltages and negative voltages, the heterojunction PD can operate in a photoconductive mode or a depletion mode, respectively. In view of the PD performance, it displays decent dark current and DUV photoresponses. At voltage of 5 and –5 V, under 254 nm DUV light illumination, the photoresponsivity (<i>R</i>) is 2.09 and 66.32 mA/W, respectively, while under 365 nm DUV light illumination, <i>R</i> is 0.22 and 34.75 mA/W, respectively. In addition, under the built-in electric field (<i>E</i><sub>built-in</sub>), <i>R</i> is 0.13 and 0.01 mA/W for 254 nm and 365 nm DUV light illumination, respectively. In all, the fabricated heterojunction PD displays promising prospects in the coming next-generation semiconductor photodetection technology. The results in this work indicate the potential of Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction with high performance DUV photodetection. Furthermore, except for the characterizations of the materials and photodetector, in the end of this paper, the operating mechanism of the dual-band dual-mode heterojunction PD is analyzed through its heterogeneous energy-band diagram. It is concluded that the illustrated dual-band dual-mode Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction can be sensitive to UVA waveband and UVC waveband in the electromagnetic spectrum, extending its photodetection region. And, the dual-mode (photoconductive mode and depletion mode) photodetection indicates two kinds of carrier transports in one PD, which can be attributed to the successful construction of the N-N tomo-type Ga<sub>2</sub>O<sub>3</sub>/Al<sub>0.1</sub>Ga<sub>0.9</sub>N heterojunction.