Patent Publication Number: US-6664561-B2

Title: Light-receiving device with quantum-wave interference layers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of Ser. No. 09/300,389 (“the parent application”) filed Apr. 27, 1999 (now allowed) and claims priority to Japanese Application No. JP 10-134335 filed Apr. 28, 1998. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an opto-electric conversion device with a new structure, or a light-receiving device. 
     2. Description of the Related Art 
     A light-receiving device has been known to have a pin junction structure. A backward voltage is applied to the pin layers of the device, and electron-hole pairs are generated by that light incident from the side of a p-layer is absorbed in an i-layer. The electron-hole pairs excited in the i-layer are accelerated by a backward voltage in the i-layer, and electrons and holes are flowing into an n-layer and a p-layer, respectively. Thus a photocurrent whose intensity varies according to an intensity of the incident light is outputted. 
     To improve an opto-electric conversion effectivity, the i-layer which absorbs light is formed to have a comparatively larger thickness. But when the thickness of the i-layer becomes thicker, more times are needed to draw carriers to the n-layer and the p-layer. As a result, the response velocity of the opto-electric conversion is lowered. To improve the velocity, an electric field in the i-layer is increased by increasing a backward voltage. But when the backward voltage is enlarged, element separation becomes difficult and a leakage current occurs. As a result, a photocurrent which flows when light is not incident on the device, or a dark current, is increased. 
     Thus conventional light-receiving devices had an interrelation among a light-receiving sensitivity, a detecting velocity, and a noise current, which restricts their performances. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to improve the light-receiving sensitivity and the response velocity Of the opto-electric conversion by providing a light-receiving device having a pin junction of a completely new structure. 
     In light of these objects a first aspect of the present invention is a light-receiving device, which converts incident light into electric current, constituted by quantum-wave interference layer units having plural periods of a pair of a first layer and a second layer, the second layer having a wider band gap than the first layer, and a carrier accumulation layer disposed between adjacent two of the quantum-wave interference layer units. Each thickness of the first and the second layers is determined by multiplying by an odd number one fourth of a quantum-wave wavelength of carriers in each of the first and the second layers, and the carrier accumulation layer has a band gap narrower than that of said second layer. Plural units of the quantum-wave interference layers are formed with a carrier accumulation layer, which has a band gap narrower than that of the second layer, lying between each of the quantum-wave interference units. 
     The second aspect of the present invention is to set a kinetic energy of the carriers, which determines the quantum-wave wavelength, at the level near the bottom of a conduction band when the carriers are electrons or at the level near the bottom of a valence band in the second layer when the carriers are holes. 
     The fourth aspect of the present invention is to define each thickness of the first and the second layers as follows: 
     
       
           D   W   =n   W λ W /4= n   W   h /4[2 m   W ( E+V )] 1/2   (1) 
       
     
     and 
     
       
           D   B   =n   B λ B /4= n   B   h /4(2 m   B   E ) 1/2   (2) 
       
     
     In Eqs. 1 and 2, h, m W , m B , E, V, and n W , n B  represent Plank&#39;s constant, the effective mass of carrier conducting in the first layer, the effective mass of carriers in the second layer, the kinetic energy of the carriers at the level near the lowest energy level of the second layer, the potential energy of the second layer relative to the first layer, and odd numbers, respectively. 
     The fourth aspect of the present invention is a quantum-wave interference layer having partial quantum-wave interference layers I k  with arbitrary periods T k  including a first layer having a thickness of n Wk λ Wk /4 and a second layer having a thickness of n Bk λ Bk /4 for each of a plural different values E k , E k +V. E k , E k +V, λ Bk , λ Wk , and n Bk , n Wk  represents a kinetic energy of carriers conducted in the second layer, a kinetic energy of carriers conducted in the first layer, a quantum-wave wavelength corresponding energies of the second layer and the first layer, and odd numbers, respectively. 
     The fifth aspect of the present invention is to form a carrier accumulation layer having the same bandwidth as that of the first layer. 
     The sixth aspect of the present invention is to form a carrier accumulation layer having a same thickness as its quantum-wave wavelength λ W . 
     The seventh aspect of the present invention is to form a δ layer between the first layer and the second layer, which sharply varies band gap energy at the boundary between the first and second layers and is substantially thinner than that of the first and the second layers. 
     The eighth aspect of the present invention is a light-receiving device having a pin junction structure, and the quantum-wave interference layer and the carrier accumulation layer are formed in the i-layer. 
     The ninth aspect of the present invention is to form the quantum-wave interference layer and the carrier accumulation layer in the n-layer or the p-layer. 
     The tenth aspect of the present invention is a light-receiving device having a pin junction structure. 
     First to Third, and Eighth to Tenth Aspects of the Invention 
     The principle of the light-receiving device of the present invention is explained hereinafter. FIG. 1 shows an energy diagram of a conduction band and a valence band when an external voltage is applied to the interval between the p-layer and the n-layer in a forward direction. As shown in FIG. 1, the conduction band of the i-layer becomes plane by applying the external voltage. Four quantum-wave interference layer units Q 1  to Q 4  are formed in the i-layer, and carrier accumulation layers C 1  to C 3  are formed at each interval of the quantum-wave interference layer units. FIG. 2 shows a conduction band of a quantum-wave interference layer unit Q 1  having a multi-layer structure with plural periods of a first layer W and a second layer B as a unit. A band gap of the second layer B is wider than that of the first layer W. 
     Electrons conduct from left to right as shown by an arrow in FIG.  2 . Among the electrons, those that exist at the level near the lowest energy level of a conduction band in the second layer B are most likely to contribute to conduction. The electrons near the bottom of the conduction band of the second layer B have a kinetic energy E. 
     Accordingly, the electrons in the first layer W have a kinetic energy E+V which is accelerated by potential energy V due to the band gap between the first layer W and the second layer B. In other words, electrons that move from the first layer W to the second layer B are decelerated by potential energy V and return to the original kinetic energy E in the second layer B. As explained above, the kinetic energy of electrons in the conduction band is modulated by potential energy due to the multi-layer structure. 
     When thicknesses of the first layer W and the second layer B are equal to an order of the quantum-wave wavelength, electrons tend to have characteristics of a wave. The wave length of the electron quantum-wave is calculated by Eqs. 1 and 2 using kinetic energy of the electron. Further, defining the respective wave number vector of first layer W and second layer B as K W  and K B , reflectivity R of the wave is calculated by: 
     
       
           R =(| K   W   |−|K   B |)/(| K   W   |+|K   B |)=([ m   W   
       
     
     
       
         ( E+V )] 1/2   −[m   B   E ] 1/2 )/([ m   W ( E+V )] 1/2   +[m   B   E]   1/2 )= 
       
     
     
       
         [1−( m   B   E/m   W ( E+V )) 1/2 ]/[1+( m   B   E/m   W ( E+V )) 1/2 ]  (3). 
       
     
     Further, when m B =m W , the reflectivity R is calculated by: 
     
       
           R =[1−( E /( E+V )) 1/2 ]/[1+( E /( E+V )) 1/2 ]  (4). 
       
     
     When E/(E+V)=x, Eq. 6 is transformed into: 
     
       
           R =(1 −x   1/2 )/(1+ x )  (5). 
       
     
     The characteristic of the reflectivity R with respect to the energy ratio x obtained by Eq. 5 is shown in FIG.  3 . 
     And when the second layer B and the first layer W have an s-layers structure, the reflectivity R S  of an incident plane of the quantum-wave is calculated by: 
     
       
           R   S =[(1 −x   s )/(1 +x   s )] 2   (6). 
       
     
     When the condition x≦{fraction (1/10)} is satisfied, R≧0.52. Accordingly, the relation between E and V is satisfied with: 
     
       
           E≦V/ 9  (7). 
       
     
     Since the kinetic energy E of the conducting electrons in the second layer B exists near the bottom of the conduction band, the relation of Eq. 7 is satisfied and the reflectivity R at the interface between the second layer B and the first layer W becomes 52% or more. Consequently, the multi-layer structure having two kinds of layers with band gaps different from each other enables reflection of quantum-wave of electrons, which conduct in an i-layer, between the first and second layers. 
     Further, utilizing the energy ratio x enables the thickness ratio D B /D W  of the second layer B to the first layer W to be obtained by: 
     
       
           D   B   /D   W   =[m   W /( m   B   x )] 1/2   (8). 
       
     
     When light is incident to the i-layer, electrons excited in conduction bands of the carrier accumulation layers C 1 , C 2  and C 3  are accumulated therein. The excited electrons tend to flow to the p-layer by applying the forward voltage, but the electrons do not flow because a reflection condition is satisfied for electrons in the quantum-wave interference layer unit which exists at the side toward the p-layer. 
     But when the electrons existing in the carrier accumulation layers C 1 , C 2  and C 3  are increased, electrons tend to exist in higher level. Then a kinetic energy of the electrons existing in higher level increases, and the electrons are not reflected by the quantum-wave interference layer units because of unsatisfaction of the reflection condition. As a result, the electrons pass the quantum-wave interference layer units Q 2 , Q 3 , and Q 4  and flow toward the p-layer, and thereby a photocurrent results. 
     Because a forward voltage is applied to the light-receiving device, driving at a low voltage becomes possible and an element separation become easier. When light is not incident, electrons are reflected in the quantum-wave interference layer units effectively. As a result, an electric current does not occur and a dark current can be substantially lowered. The present inventor thinks that electrons are conducted in the quantum-wave interference layer units as a wave. Accordingly, a response velocity is considered to become larger. 
     The thicknesses of the first layer W and the second layer B are determined for selectively reflecting the holes or the electrons, because of the difference in potential energy between the valence and the conduction bands, and the difference in effective mass of holes and electrons in the first layer W and the second layer B. In other words, the optimum thickness for reflecting electrons is not the optimum thickness for reflecting holes. Eqs. 3-8 refer to a structure of the quantum-wave interference layer for selectively reflecting electrons. The thickness for selectively reflecting electrons is designed based on the difference in the potential energy of the conduction band and on the effective mass of electrons. Further, the thickness for selectively reflecting holes is designed based on the difference in the potential energy of the valence band and on the effective mass of holes, forming another type of quantum-wave interference layer in an i-layer for reflecting only holes and allowing electrons to pass through. 
     Accordingly, a quantum-wave interference layer unit which reflects holes and functions as a reflective layer to holes can be formed to connect in series to each quantum-wave interference layer units described above, which reflects only electrons. 
     The light-receiving device described above having a quantum-wave interference layer unit can have a state not to generate an electric current by reflecting carriers selectively in a range of 0 V to a certain value of a bias voltage. Accordingly, the light-receiving device can be formed by only one of the n-layer and the p-layer in which the quantum-wave interference layer units and the carrier accumulation layer are formed. Alternatively, the light-receiving device can be formed by a pn junction structure, in which the quantum-wave interference layer units and the carrier accumulation layer are formed. 
     Fourth Aspect of the Present Invention 
     FIG. 4 shows a plurality quantum-wave interference units I k  with arbitrary periods T k  including a first layer having a thickness of D Wk  and a second layer having a thickness of D Bk  and arranged in series. 
     Each thickness of the first and the second layers satisfies the formulas: 
     
       
           D   Wk   =n   Wk λ Wk /4 =n   Wk   h /4[2 m   Wk ( E   k   +V )] 1/2   (9) 
       
     
     and 
     
       
           D   Bk   =n   Bk λ Bk /4 =n   Bk   h /4(2 m   Bk   E   k ) 1/2   (10) 
       
     
     In Eqs. 9 and 10, E k , m Wk , m Bk , and n Wk  and n Bk  represent plural kinetic energy levels of carriers conducted into the second layer, effective mass of carriers with kinetic energy E k +V in the first layer, effective mass of carriers with kinetic energy E k  in the second layer, and arbitrary odd numbers, respectively. 
     The plurality of the partial quantum-wave interference layers I k  are arranged in series from I l  to I j , where j is a maximum number of k required to form a quantum-wave interference layer as a whole. The carriers existing in a certain consecutive energy range can be reflected by narrowing discrete intervals. 
     Fifth and Sixth Aspects of the Present Invention 
     The fifth aspect of the present invention is to form the bandwidth of the carrier accumulation layer to have the same bandwidth as that of the first layer. And the sixth aspect of the present invention is to form the carrier accumulation layer to have a same thickness as its quantum wave wavelength λ W . As a result, the carriers excited in the i-layer can be confined effectively. 
     Seventh Aspect of the Present Invention 
     The seventh aspect of the present invention is to form a δ layer at the interface between the first layer W and the second layer B. The δ layer has a thickness substantially thinner than both of the first layer W and the second layer B and sharply varies the energy band profile of the device. The reflectivity R of the interface is determined by Eq. 5. By forming the δ layer, the potential energy V of an energy band becomes larger and the value x of Eq. 5 becomes smaller. Accordingly, the reflectivity R becomes larger. 
     Variations are shown in FIGS. 5A to  5 C. The δ layer may be formed on both ends of every first layer W as shown in FIGS. 5A to  5 C. In FIG. 5A, the δ layers are formed so that an energy level higher than that of the second layer B may be formed. In FIG. 5B, the δ layers are formed so that an energy level lower than that of the first layer W may be formed. In FIG. 5C, the δ layers are formed so that a band bottom higher than that of the second layer B and a band bottom lower than that of the first layer W may be formed. As an alternative to each of the variations shown in FIGS. 5A to  5 C, the δ layer can be formed on one end of every first layer W. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and characteristics of the present invention will become apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of the specification, and wherein reference numerals designate corresponding parts in the various figures, wherein: 
     FIG. 1 is a view showing the energy diagram of a quantum-wave interference layer according to the present invention; 
     FIG. 2 is an explanatory view of a conduction band of a multi-layer structure of the present invention; 
     FIG. 3 is a graph showing a relation between an energy ratio x and a reflectivity R; 
     FIG. 4 is an explanatory view of partial quantum-wave interference layers I k ; 
     FIGS. 5A to  5 C are explanatory views of δ layers according to the present invention; 
     FIG. 6 is a sectional view showing a first exemplary structure of a semiconductor device  100 ; 
     FIG. 7 is a graph showing V-I characteristics of the light-receiving device  100  when light is incident and not incident; 
     FIG. 8 is an explanatory view showing a structure of a light-receiving device  200 ; 
     FIG. 9 is a graph showing V-I characteristic of the light-receiving device  200  when light is incident and not incident; 
     FIG. 10 is an explanatory view showing a structure of a light-receiving device  300 ; and 
     FIG. 11 is a graph showing V-I characteristic of the light-receiving device  300  when light is incident and not incident. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be more fully understood by reference to the following examples. 
     FIG. 6 is a sectional view of a semiconductor device  100  having an pin junction structure in which a quantum-wave interference layer is formed in an i-layer. The light-receiving device  100  has a substrate  10  made of gallium arsenide (GaAs). A GaAs buffer layer  12  of n-type conduction, having a thickness generally of 0.3 μm and an electron concentration of 2×10 18 /cm 3 , is formed on the substrate  10 . An n-Ga 0.51 In 0.49 P contact layer  14  of n-type conduction, having a thickness generally of 0.13 μm and electron concentration of 2×10 18 /cm 3 , is formed on the buffer layer  12 . An n-Al 0.51 In 0.49 P n-layer  16  of n-type conduction, having a thickness generally of 0.2 μm and an electron concentration of 1×10 18 /cm 3 , is formed on the contact layer  14 . A non-doped i-layer  18  is formed on the n-layer  16 . An Al 0.51 In 0.49 P p-layer  20  of p-type conduction, having a thickness generally of 0.2 μm and a hole concentration of 1×10 18 /cm 3 , is formed on the i-layer  18 . A p-Ga 0.51 In 0.49 P second contact layer  22  of p-type conduction, having a thickness generally of 0.13 μm and a hole concentration of 2×10 18 /cm 3 , is formed on the p-layer  20 . A p-GaAs first contact layer  24  of p-type conduction, having a thickness generally of 0.06 μm and a hole concentration of 2×10 18 /cm 3 , is formed on the second contact layer  22 . An electrode layer  26  made of gold and germanium (Au/Ge), having a thickness generally of 0.2 μm, is formed so as to cover the entire back of the substrate  10 . Another electrode layer  28  made of Au/Zn, having a thickness generally of 0.2 μm, is formed on some portion of the first contact layer  24 . 
     A quantum-wave interference unit Q 1  having a multi-quantum layer structure with 10 pairs of a Ga 0.51 In 0.49 P first layer W, having a thickness of 5 nm, a Al 0.51 In 0.49 P second layer B, having a thickness of 7 nm, and a non-doped Al 0.33 Ga 0.33 In 0.33 P δ layer, having a thickness of 1.3 nm, sandwiching the first layer W is formed in the i-layer  18 . Q 2 , . . . Q 4  are formed like Q 1  and  4  quantum-wave interference units in total are formed in the i-layer  18 . FIG. 5 shows a band structure of the quantum-wave interference layer Q 1  in detail. Non-doped Ga 0.51 In 0.49 P carrier accumulation layers C 1  to C 3 , having a thickness of 20 nm, are formed between any quantum-wave interference units Q 1  and Q i+l , respectively. Thicknesses of the first layer W and the second layer are determined according to Eqs. 1 and 2, respectively, on condition that a forward voltage is applied to the interface between the electrodes  28  and  26 , and that no electric potential gradient is occurring in the i-layer  18 . 
     The second layers B which contact to the n-layer  16  and the p-layer  20  have a thickness of 0.05 μm, respectively. They are formed thicker than other second layers to prevent a tunneling conduction of carriers from the n-layer  16  or the p-layer  20  to the first layer W. And the substrate  10  has a diameter of 2.0 inches and the normal direction of its main surface is offset toward the [011] axis by 15 degree from the (100) plane. 
     The light-receiving device  100  was manufactured by gas source molecular beam epitaxial deposition (GS-MBE) which is an epitaxial growth method under an extremely high vacuum condition. GS-MBE is different from a conventional MBE which supplies group III and V elements both from solid state sources. In GS-MBE, group III elements such as indium (In), gallium (Ga), and aluminum (Al) are supplied from a solid source and group V elements such as arsenic (As) and phosphorous (P) are supplied by heat decomposition of gas material such as AsH 3  and PH 3 . Alternatively, the light-receiving device  100  can be manufactured by metal organic chemical vapor deposition (MOCVD). 
     As shown in FIG. 1, as a forward voltage V applied to the interface between the p-layer  20  and the n-layer  16  of the light-receiving device  100  increases, an electric potential gradient occurring in the i-layer  18  becomes gentler until it becomes planar. In this condition, electrons do not flow because a reflection condition to electrons in quantum-wave interference layers Q 1  to Q 4  is satisfied. 
     When light having an energy resonant to the bandwidth of carrier accumulation layers C 1  to C 3  is incident, electrons are excited in the carrier accumulation layers C 1  to C 3 . An electron concentration in the carrier accumulation layers C 1  to C 3  becomes larger, and many electrons come to exist at levels higher than the bottom of a conduction band in the second layer B. Then electrons in the n-layer  16  are conducted into the carrier accumulation layers C 1  which is adjacent to the n-layer  16 , and electrons in the carrier accumulation layers C 1  are conducted into the carrier accumulation layers C 2 . Accordingly, electrons intervene each carrier accumulation layers C 1  and are conducted to each carrier accumulation layers at a high speed, by wave propagation of electrons as a wave. Thus electrons are conducted from the n-layer  16  to the p-layer  20  by a light excitation at a high speed. 
     The light-receiving device  100  has a high opto-electric conversion effectivity because electrons, which are excited in the carrier accumulation layers C 1  to C 3 , function as a gate-controlled switch toward the conduction of electrons from the n-layer  16  to the p-layer  20 . When electrons are not excited in the carrier accumulation layers C 1  to C 3 , a condition to reflect electrons is satisfied in the quantum-wave interference layers Q 1  to Q 4 . But when electrons are excited in the carrier accumulation layers C 1  to C 3 , the condition is not satisfied and electrons may be conducted in the quantum-wave interference layers Q 1  to Q 4  as a wave. Accordingly, a switching velocity is considered to be larger. 
     Measured V-I characteristics of the light-receiving device  100  are shown in FIG.  7 . When light is incident, the photocurrent rises abruptly from 10 −11  to 10 −7 , or in the range of 4 orders, at the forward voltage of 0.2 V. But even if a forward voltage is applied to the device, a dark current is suppressed at a lower value and degree of increasing is also suppressed. The phenomenon occurs because the dark electrons reflected by the quantum-well interference layers and the dark current is kept comparatively lower. And the photocurrent when light is incident on the diode, represented by Al, is about one hundredfold that of a dark current, represented by B 1 . Additionally, the forward applied voltage at which an electric potential gradient in the i-layer  18  becomes planar appears to be 1 V. When an applied forward voltage is 1 V, the photocurrent is not less than 5×10 −6  A. 
     A light-receiving device  200  having an nipin structure shown in FIG. 8 is formed. The regions represented by a 1 , a 2 , and a 3  are the same as those in the light-receiving device  100  having an nip structure. A quantum-wave interference layer and a carrier accumulation layer are formed in an i-layer a 2 . Regions a 4  and a 5  function to draw the photocurrent which flows into the p-type region a 3  by a reverse external bias voltage. The V-I characteristic was measured when the light-receiving device  200  is incident by light. As shown in FIG. 9, the photocurrent, represented by A 2 , is about one thousandfold that of a dark current, represented by B 2 . 
     A light-receiving device  300  using Si/Ge compound semiconductor shown in FIG. 10 is formed. A first layer W made of Si 0.8 Ge 0.2  and a second layer B made of Si are formed to have thicknesses of 5 nm and 7 nm, respectively. Carrier accumulation layers C 1  to C 3  made of Si 0.8 Ge 0.2  are formed to have a thickness of 20 nm. No δ layer is formed in the device  300 . 
     A characteristic of the device  300  was measured when the device is incident by light. As shown in FIG. 11, the photocurrent, represented by A 3 , is one thousandfold that of a dark current, represented by B 3 . When the estimated forward voltage between n-layer a 1  and p-layer a 3  is about 0.9 V, the photocurrent rises abruptly. Further, by forming a δ layer, the characteristic of the light-emitting device  300  will be improved. 
     Accordingly, the light-receiving device of the present invention can obtain a larger S/N ratio compared with conventional devices. 
     In the embodiment, a δ layer is formed in the devices  100  and  200 . The δ layer improves the reflectivity of the devices  100  and  200 . Alternatively, because the reflectivity can be improved by a multipath reflection, the δ layer is not necessarily needed. 
     In the embodiment, four quantum-wave interference units Q 1  to Q 4  are connected in series, with a carrier accumulation layer C lying between each of the quantum-wave interference units. Alternatively, an arbitrary number of the quantum-wave interference units can be connected in series. 
     In the first and second embodiments, the quantum-wave interference layer was formed to have a multi-layer structure including Ga 0.51 In 0.49 P and Al 0.51 In 0.49 P. Alternatively, quantum-wave interference layer can be made of quaternary compounds such as a general formula Al x Ga y In l-x-y P, selecting an arbitrary composition ratio within the range of 0≦x≦1, 0≦y≦1, and 0≦x+y≦1. Further alternatively, the quantum-wave interference unit can be made of a pair of group III-V compound semiconductors with different composition ratios, a pair of group II-VI compound semiconductors with different composition ratios, a pair of Si and Ge, and semiconductors of other hetero-material. 
     While the invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.