JP2773227B2 - Photo conductor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a photoconductor, and more particularly, to a structure of a photoconductor capable of improving its response speed.

[Conventional technology]

FIG. 5 shows an example of a conventional photoconductor. Such a photoconductor is known as a Physics of Semiconductor device.
or Devices), page 744, reported by Sze. An active layer 53 made of non-doped GaAs is formed on a semi-insulating GaAs substrate 1, and ohmic electrodes are provided at both ends of the active layer 53.
A1 and A2 are formed. When light having energy equal to or greater than the band gap of GaAs is incident on the upper surface of the active layer 53 with a voltage applied between the electrodes A1 and A2, electron-hole pairs are generated and a photocurrent flows.

[Problems to be solved by the invention]

The gain (Gain) and the response time (tr) in the photoconductor are represented by the following equations.

Gain = μ _n · τ · E / L (1) _tr = L / (μ _n · E) (2) where μ _n is carrier mobility, τ is carrier lifetime,
E is the electric field, and L is the electrode spacing. Here, in the above-mentioned conventional photoconductor in which the active layer is formed from a non-doped semiconductor, a long carrier life is obtained and thus a high gain is obtained. There was a problem that the response time was long in comparison.

SUMMARY OF THE INVENTION An object of the present invention is to provide a photoconductor that solves this problem and that can shorten the response time while maintaining a high gain.

[Means for solving the problem]

A photoconductor according to a first aspect of the present invention includes, as an active layer, a first semiconductor layer having a conductivity of 1 × 10 ¹⁷ / cm ³ or less and having a thickness of about the de Broglie wavelength of a carrier. Is formed on a second semiconductor layer which is non-doped and has a larger band gap than the first semiconductor layer, and has an impurity concentration of 1 in the first semiconductor layer. × 10
Opposite conductivity type regions of ¹⁷ / cm ³ or less are formed in a striped arrangement with an interval of about the de Broglie wavelength of the carrier, and furthermore, electrodes facing each other in the longitudinal direction of the stripe-shaped opposite conductivity type regions are formed. It is characterized by being provided.

According to a second aspect of the present invention, there is provided a photoconductor in which a first semiconductor layer which is non-doped and has a thickness of about the de Broglie wavelength of a carrier and a second semiconductor layer which is non-doped and has a band gap larger than that of the first semiconductor layer are alternately formed. An active layer comprising a superlattice layer formed by lamination, the active layer comprising the superlattice layer being formed on a third semiconductor layer which is non-doped and has a larger band gap than the first semiconductor layer; In the active layer composed of the superlattice layer, non-doped semiconductor regions having a bandgap larger than that of the first semiconductor are formed in a stripe pattern at intervals of about the de Broglie wavelength of carriers. Characterized in that an opposed electrode is provided in the longitudinal direction of the non-doped semiconductor region.

[Action]

As reported by Sakaki in Japan Journal of Applied Physics (Jpn. J. Appl. Phys.), Vol. 19, p. L735, 1980, one-dimensional conduction blocks small-angle scattering, At low temperatures, where ionized impurity scattering is dominant, the low field mobility of electrons is expected to reach 10 ⁶ cm ² / Vs or more. In addition, since the one-dimensional density of states decreases with energy, electrons accelerated to higher than the optical phonon energy in a high electric field are less likely to be scattered as they are accelerated, and the drift velocity increases rapidly. Yamada et al., IEICE Technical Report, ED87-99, pp. 15, 1987.

According to the present invention, the carrier mobility is improved and the response time of the photoconductor is remarkably reduced by forming the active layer, which is conventionally formed of a bulk semiconductor, with a one-dimensional conduction channel based on such a principle. Things. That is, if the active layer of the photoconductor is constituted by a one-dimensional conduction channel that can provide a high mobility, the response time can be reduced according to the equations (1) and (2), and a higher gain can be achieved. Here, as the one-dimensional channel, there is a method of forming a fine line by etching. However, in the case of thin wires, channel depletion occurs due to surface potential, making it difficult to control the channel width, and problems such as the influence of surface levels are conceivable.

Therefore, in the present invention, the surface superlattice is formed by forming the regions of the opposite conductivity type with the low carrier concentration in the thin film channel layer with the low carrier concentration in the form of stripes at a pitch of about the electron de Broglie wavelength. I have. With such a structure, the influence of the surface potential and the surface level can be minimized. Although the light carriers are excited in the entire surface superlattice, electrons n by an internal electric field ^- the hole in the region
The quasi-one-dimensional channels are formed because they are collected in the p ^- region and travel through channels each having a cross-sectional shape of about the de Broglie wavelength. In addition, since the carrier concentration in the backround in the surface superlattice is 1 × 10 ¹⁷ / cm ³ or less, which is sufficiently lower than that of the photoexcited carriers, a decrease in detection accuracy due to dark current is avoided. Here, in order to sufficiently bring out the effect of one-dimensional conduction, it is desirable that incident light be absorbed only by the one-dimensional channel and that no contribution of conduction of the bulk layer be made. Therefore, the bulk layer below the one-dimensional channel is formed of a semiconductor having a band gap larger than that of the active layer. As incident light, the energy is lower than the band gap of the bulk layer and higher than the band gap of the active layer. By using this light, the influence of the bulk layer can be eliminated.

In addition, the one-dimensional channel has a high mobility, but has a problem that a current amount per unit channel width is small. This is because the cross-sectional area of the one-dimensional channel is limited to about 100 ° × 100 °, and this problem can be solved by increasing the number of one-dimensional channels. However, increasing the number of channels leads to an increase in element size,
Not desirable. Therefore, by changing the active layer from a single-layer thin film to a superlattice layer, the current can be increased while preventing an increase in element size. In the superlattice structure, each quantum well constitutes a one-dimensional channel, and the quantum barrier controls the exudation of carriers between the quantum wells to ensure one-dimensionality of the channel.

Further, there is another merit by making the active layer have a superlattice structure. For example, as reported by Uematsu et al. In IEICE Technical Report, Vol. ED-87, No. 92, p. 15, if the superlattice structure is mixed-crystallized by ion implantation, the band gap Is larger than the quantum well.
In a photoconductor in which a channel is formed by forming a mixed crystal region in a stripe shape by this method, 1
Since the dimensional channel is surrounded on all sides by the quantum barrier layer having a larger band gap and the mixed crystal region, the optical carrier is excited only in the one-dimensional channel portion, and the effect of confining the carrier is further improved.

〔Example〕

1 (a) and 1 (b) show an element structure of a photoconductor of an embodiment according to the first invention. Semi-insulating GaAs substrate 1
A non-doped Al _0.4 Ga _0.6 As buffer layer 2 is formed thereon with a thickness of 1 μm, and an active layer 3 of n ^- GaAs with an impurity concentration of 1 × 10 ¹⁶ / cm ³ having a thickness of about 200 ° is formed on the surface thereof. I have. The active layer 3 has an impurity concentration of 1.times.
A p ^- region 3I of about 10 ¹⁶ / cm ³ is formed in a stripe shape at intervals of 200 °. At both ends of the active layer 3, ohmic electrodes A1, A
Two are provided. Figure 1 (b) is a device sectional view of _{x 1 -x 2 -x 3 -x 4} -x 1 side of the embodiment shown in FIG. 1 (a). Here, when light having a wavelength of not less than 0.65 μm and not more than 0.85 μm is incident from the upper surface of the element, the incident light is absorbed only in GaAs, and carriers are excited in the active layer 3 and flow through the channel. Here, since the incident light is not absorbed by the AlGaAs layer 2, only the flow can be extracted between the electrodes A1 and A2 by the light in the active layer 3.

Such a photoconductor is manufactured by a manufacturing method described below. A non-doped AaGaAs layer 2 is formed on a semi-insulating GaAs substrate 1 by using, for example, a molecular beam epitaxial growth method (MBE method) to a thickness of 1 μm and an impurity concentration of 1 × 10 5.
^{A 16} / cm ³ n ^- GaAs layer 3 is sequentially grown at 200 °. Next, after forming a resist pattern, Ga ions are implanted at a concentration of 2 × 10 ¹⁰ / cm ² by using a focused ion beam exposure (FIB) method, so that p ⁻
The region 3I is formed. Finally, if the ohmic electrodes A1 and A2 to the channels are formed by an ordinary method, the photoconductor as in this embodiment is manufactured.

FIG. 2 is a band structure diagram of a cross section in the channel width direction (y ₁ -y ₂ in FIG. 1B) of the active layer of the photoconductor according to the present embodiment. In the figure, reference numeral 3 denotes a one-dimensional channel of electrons in n ^- GaAs, and 3I denotes a p ^- region. Ec indicates the lower end of the conduction band, and Ev indicates the upper end of the valence band. As shown, a surface superlattice structure is formed in the active layer in the channel width direction.
While excited with (both areas n ^{- -} GaAs and p), the electrons to the internal electric field n ^- photocarriers entire active layer in the portion 3 of the hole is p ^- is collected in the portion 3I, respectively Run in a one-dimensional channel. Since the cross section of the channel has a dimension on the order of the de Broglie wavelength of electrons, carriers behave one-dimensionally, and high mobility and high drift velocity are realized based on the principle as described in the section of [Action].

FIGS. 3A and 3B show an element structure of a photoconductor according to an embodiment of the second invention. Semi-insulating GaAs substrate 1
A non-doped Al _0.4 Ga _0.6 As buffer layer 2 is formed thereon with a thickness of 1 μm, and an active layer 33 composed of a superlattice of a non-doped GaAs layer 33A and a non-doped Al _0.4 Ga _0.6 As layer 33B is formed on the surface. In the active layer 33, non-doped Al _0.2 Ga _0.8 As regions 33I having a width of several hundreds of squares are formed in stripes at intervals of 200 °. Ohmic electrodes A1 and A2 are formed at both ends of the active layer 33. Figure 3 (b) is a device sectional view of _{x 1 -x 2 -x 3 -x 4} -x 1 side of the embodiment shown in FIG. 3 (a). Superlattice active layer 33 is a non-doped GaAs layer
It is composed of a laminated structure of 33A and a non-doped Al _0.4 Ga _0.6 As layer 33B, the thickness of the undoped GaAs layer 33A is about 200 Å, an undoped Al _0.4 Ga _0.6 As layer 33B also is about 200 Å.

Such a photoconductor is manufactured by a manufacturing method described below. On a semi-insulating GaAs substrate 1, a non-doped AlGaAs layer 2 is formed to a thickness of 1 μm using a molecular beam epitaxial growth method (MBE method).
A superlattice layer 33 composed of a stack of 200 Å and a non-doped Al _0.4 Ga _0.6 As layer 200 順次 is sequentially grown. Next, after a resist pattern is formed, Ga ions are implanted at 1 × 10 ¹⁵ / cm ² using focused ion beam exposure (FIB method), and lamp annealing is performed to crystallize the superlattice layer in the stripe region. As a result, a non-doped Al _0.2 Ga _0.8 As region 33I is formed. Finally, if the ohmic electrodes A1 and A2 to the channels are formed by an ordinary method, the photoconductor as in this embodiment is manufactured.

FIG. 4 is a band structure diagram in the active layer of the present embodiment. FIG. 4A shows a cross section in the channel width direction (y ₁ -y _{2 in} FIG. 3B) in the quantum well layer of the superlattice, and FIG. 4B shows the channel in the quantum barrier layer of the superlattice. shows a cross section (y ₃ -y ₄ of FIG. 3 (b)) width direction. In the figure, 33
A is a quantum well layer constituting a one-dimensional channel, 33B is a quantum barrier layer, and 33I is an Al _0.2 Ga _0.8 As region. The channel layer 33A (bandgap ~ 1.42eV) is larger than the quantum barrier layer 33B (bandgap ~ 1.92eV).
V) and the high-resistance region 33I (band gap: 1.67 eV). Where 0.
When light having a wavelength of not less than 75 μm and not more than 0.85 μm is incident from the upper surface of the element, the incident light is absorbed only in GaAs and transmitted in AlGaAs without absorption, so that carriers are excited only in each channel layer 33A. Each of the AlGaAs layers 33B and 33I has a thickness sufficient to suppress the exudation of carriers.
Each of the GaAs layers 33A forms an independent pseudo one-dimensional channel, and carriers travel in the one-dimensional channel confined by the heterojunction barrier. The cross section of each GaAs layer 33A is
Since both the width and the thickness have dimensions about the electron de Broglie wavelength, the carriers behave one-dimensionally, realizing high mobility and high drift velocity.

In each of the above embodiments, the description has been made using a GaAs / AlGaAs / GaAs-based photoconductor.
It can be applied to various material systems such as As / GaInAsP / InP system and GaInAs / AlInAs / InP system.

〔The invention's effect〕

As is clear from the above description, according to the present invention, a photoconductor having a quasi-one-dimensional channel as an active layer can be obtained, so that the gain can be improved and the response speed can be significantly improved. If the channel has a superlattice structure, a structure that can increase the current and improve the confinement of carriers can be easily obtained.

[Brief description of the drawings]

1 (a) and 1 (b) are element structure diagrams of an embodiment of the photoconductor according to the first invention, FIG. 2 is a band structure diagram of the photoconductor of FIG. 1, and FIGS. 3 (a) and 3 (b) ) Is an element structure diagram of an embodiment of the photoconductor according to the second invention, FIG. 4 is a band structure diagram of the photoconductor of FIG. 3, and FIG. 5 is an element structure diagram of an example of a conventional photoconductor. 1 ... Semi-insulating GaAs substrate 2 ... Non-doped Al _0.4 Ga _0.6 As buffer layer 3 ... n ^- GaAs layer 3I ... P ^- GaAs region A1, A2 ... Ohm electrode 33 ... Superlattice layer 33A ... Non-doped GaAs layer 33B Non-doped Al _0.4 Ga _0.6 As layer 33I Non-doped Al _0.2 Ga _0.8 As region 53 Non-doped GaAs layer

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 31/08 H01L 31/10 H01S 3/18

Claims (2)

(57) [Claims] An active layer comprising a first semiconductor layer of one conductivity type having an impurity concentration of 1 × 10 ¹⁷ / cm ³ or less and having a thickness of about the de Broglie wavelength of a carrier as an active layer. becomes active layer, with which is formed on the first semiconductor layer than the band gap larger second semiconductor layer on the non-doped, the impurity concentration in the active layer formed of the first semiconductor layer 1 × 10 ¹⁷ / The opposite conductivity type area of cm ³ or less is
A photoconductor characterized by being formed in a stripe pattern with an interval of about the Broy wavelength, and further provided with electrodes facing each other in the longitudinal direction of the opposite conductivity type region of the stripe pattern. 2. A non-doped first semiconductor layer having a thickness of about the de Broglie wavelength of a carrier and a non-doped second semiconductor layer having a band gap larger than that of the first semiconductor layer are alternately laminated. An active layer comprising a superlattice layer as an active layer, and
In the active layer formed of the superlattice layer, a non-doped semiconductor region having a band gap larger than that of the first semiconductor is formed on the third semiconductor layer having a band gap larger than that of the first semiconductor layer. -A photoconductor characterized in that it is formed in a stripe pattern with an interval of about the Broy wavelength, and furthermore, an electrode facing the longitudinal direction of the striped non-doped semiconductor region is provided.